Engine Knocking Sound Under Car When Accelerating (Causes & Troubleshooting)

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engine noise

So you have a mysterious knocking noise under your car every time you accelerate. Don’t panic—there are only a few things that can potentially be the cause. If you’re very concerned, get your car checked out. 

But first, read through why your car sounds like it’s dying (it probably isn’t, and if it is, it’s probably fixable).

What Causes Engine Knocking When Accelerating?

car makes noise

Let’s start with the basics of how ignition works in normal combustion. The engine burns the air/fuel mix repeatedly. At the same time, the spark plug sparks 10 to 40 degrees before TDC (top dead center). A kernel of flame is produced by the spark across the spark plug’s electrodes. This kernel grows in size and temperature. Its heat and pressure raise the temperature of the surrounding air to near its ignition point. The hot, high-pressure gases generated by the combustion (also known as flame front) then spread out across the cylinder producing a pressure wave that forces the piston down.

Engine knocking sound can be caused by a variety of factors. When accelerating from a stop, you may hear the banging noise, or when decelerating under a vehicle, you may hear a knocking sound. Let’s look at some of the most common causes of engine knocking:

Too Low Octane

Every engine is built with a specific number of octane levels in mind. The tolerance of fuel to pressure increase is determined by the octane number. The higher the octane number, the farther the piston moves toward the spark plug, resulting in greater power output. To reach its maximum level of travel, make numerous pockets of exploded air/fuel mixtures before the piston if the octane number is low. As a consequence, this results in a knocking sound as well as reduced vehicle performance.

Lean Air-Fuel Mixture

If your lean air/fuel proportion is high, it implies that the fuel proportion is too small compared to the amount of air in the AFM. When there are issues with engine management sensors such as oxygen sensors, spark plugs, fuel injectors, and mass airflow sensors of the fuel pump, this can occur. The burning duration of each mixture will be incorrect if there isn’t enough fuel in each cylinder; as a result, the mixture will detonate at an inappropriate moment. This eventually leads to the formation of some detonations, which causes the engine to knock.

Bad Knock Sensor

Most modern vehicles are equipped with an engine knock sensor detect which communications with the ECU to diagnose any problems. It’s possible that the knock sensor is defective which means it sends incorrect signals to the ECU. If this happens, the computer will start to use fuel with an air/fuel mixture proportion higher than required, and as a result, knocking occurs.

Faulty Spark Plugs

Spark plugs are necessary for the burning air/fuel mixture to generate enough power for the engine to operate. Spark plugs age and become damaged over time, resulting in reduced spark. The sparks may be delayed at times, causing the engine to produce a knocking noise when accelerating.

Carbon Deposits

Carbon cleaners are used in all forms of vehicle. They assist to prevent carbon deposits from accumulating and clogging the cylinders. When carbon clogs or builds up in the cylinder, there is less room for air/fuel to dwell, resulting in engine knocking.

Low Oil Level

Another cause could be due to a lack of oil in the engine. When the level of oil drops below a certain point, the rod bearing between the crankshaft and piston rod fractures. As it moves up and down, it makes a rattling sound.

Why Is My Car Making Noise? 

avoid vehicle engine knocking

There are so many reasons for a car to make noise. 

Old or new, you’re inevitably going to have a car making a loud noise. Even if you take excellent care of your vehicle—you never leave the check engine light on, you have a standing appointment with yourself to change your oil, and you’ve never left the door open all night—you will still run into problems. 

You shouldn’t be afraid when it happens, but you should be ready to have it repaired. 

Weird noises aren’t great. They tend to be an indication that something has gone wrong. It doesn’t always mean a major repair is on the way—there are many reasons for your car to make sounds. If you hear a knocking sound under your car when accelerating, it’s probably not terrible, but it is time for a check-up. 

Just as it could be a minor problem, it could also be a costly one or even mean the end of your car. We’re going to help you find out which thing it’s most likely to be so that you can take care of it. 

Engine Knocking

knocking sounds

If you’re noticing an engine knocking sound, there are a few common causes. The most easily fixable is low-quality fuel, which can actually cause a strange noise every time you use your car. You can check this pretty easily: Change your fuel. If the noise stops when you do this, congratulations, your car is fixed. 

A rod knock can also cause noises around the engine—this is a very serious problem. Connecting rod bearing failure could end up seizing your engine. 

Fixing or replacing an engine is very expensive, often in the thousands, and you might want to sell your vehicle instead. If changing fuel isn’t the fix for the noise, get your engine looked at by a professional. 

Clunking Noise When Braking

You may have noticed a knocking sound under your car when accelerating, but have you noticed anything when decelerating? If you hear a clunking noise when you use the brakes, or anytime you adjust your cruising speed, then you have a big problem.

A loud thumping or clunking when braking can be caused by many different things. A seized brake caliper, a very loose wheel bearing, or even worn-out ball joints and steering components can cause persistent noise when you’re using your brakes.

Any one of these problems puts you at serious risk for accident or injury. Either your brakes just aren’t working well, or your wheels aren’t moving in the exact direction you expect them to. 

Whichever problem you’re having, it’s bad. If this sounds like a problem you’re experiencing, get your brakes checked immediately. You may need something as simple as new brake pads, or you may need a completely revamped suspension and steering system. No matter the cost, it’s a necessary fix to avoid a major disaster. 

Random Car Noise at High Speeds

Maybe you only hear the knocking sound on the highway. Is it a knock? A flapping? A rattling? If one of these sounds seems closer to what you’re experiencing, this could mean that something is loose in your vehicle. 

Several things could be loose. It could be your bumper cover or trim. Some people have reported rattling noises in convertibles, and it turns out to be a poorly attached roof. 

Figure out exactly what part of your car you hear the rattling in, and also when. If it’s a flapping sound under the hood, that’s the sound of a faulty fan belt. Is it under the car? It might be a loose wheel liner. 

If it happens all the time, you probably have something moving around, likely something that’s poorly attached to the body. If it’s ONLY at certain speeds, you can have an internal problem. 

Transmission Knocking When Accelerating

If you specifically have a loud knocking when accelerating, the most likely culprit is a deteriorating driveshaft coupling. Does the knocking sound start when you reach the highway? Does it get louder the faster you go? 

Steel couplings attach your transmission and driveshaft held together with rubber doughnuts. The more the rubber deteriorates, the louder knocking you’ll have. 

If you accelerate hard and often, they’ll break down even faster. A weekend transmission mount might also cause the sound.

Knocking Sound Under Your Car on Bumpy Roads

Are you sure the knocking noise under your car is directly related to acceleration? Do you notice it equally or more when going over a speed bump or poorly paved roads? This could mean that you have a suspension issue. 

If you only hear the knocking on uneven roads, then it’s not too much of a problem—yet. Now is the time to get this taken care of. When your suspension, stabilizer links, control arms, etc., finally break down, that’s when you have a significant problem. You could end up completely losing control of your car. 

Get to a mechanic while it’s still a mysterious knocking sound and not a major accident.  

Knocking When Starting Your Car

Do you hear a clunking or knocking when you first start your car? The most likely cause is a problem with the transmission’s flexplate or the starter. 

Unfortunately, the knocking noise when starting will likely be caused by one of these things. It’s a problem that needs fixing before you have a bigger problem—and it won’t come cheap. 

Replacing the starter can usually be done in a day and can cost a few hundred dollars. But, replacing the flexplate means pulling out the whole transmission. This process can cost you upwards of $1000. 

Knocking Noise When Accelerating

So we’ve gone over a lot of reasons for knocking noises that you could have. The source of the sound makes a big difference in diagnosing the problem that you have. And the exact sound (rattling, grinding, clunking, knocking) also can mean different things. 

Again, if the knocking noise under your car only happens with acceleration, it’s most likely an engine problem, and it needs to be taken care of right away. 

If the sound comes ONLY with accelerating, that’s because of strain being put on your engine when you accelerate. Low oil pressure or piston slap is likely the cause, but there could be other things happening simultaneously. 

FAQs

Can You Drive a Car With a Knocking Engine?

Yes, your automobile may be driven with a knocking engine, but you must be cautious when operating it. While driving, try to ease up on the throttle and examine the problem after you’ve reached your destination.

What Does a Knocking Engine Sound Like?

A knocking engine has a distinct sound that varies from car to car. While some cars may have a more subtle or dull noise, others can be very loud and noticeable (ie. loud rumbling noise, rattling noise). Automobiles with modified exhaust systems or components on the engine may produce a louder sound than those without modifications.

How Much Does It Cost to Fix a Knocking Engine?

It is not difficult to repair a knocking engine. You will be required to change the spark plugs from time to time, which will add to the costs. In most situations, it runs approximately $200. If the knocking sound persists even after changing the spark plugs, you can expect to spend more money as the engine will have to be disassembled.

Will an Oil Change Stop Engine Knocking?

You have to distinguish between engine noise caused by a low oil level or pressure and the knocking sound that comes from the combustion chambers. The knocking sound is a result of problems with ignition and fuel. As a result, changing the oil will stop the cluttering noise from the valves in the cylinder head, but not the knocking sound.

Can Thin Oil Cause Knocking?

Between the crankshaft and piston rods, old engines frequently have worn-out engine bearings. When the engine bearings wear down, they create a rattling sound every time the piston rises or falls. This is known as crankshaft knock. As a result, applying thick oil to these gaps will aid in preventing noises by blocking them.

What are the symptoms of a rod knock?

Do you hear a knocking sound coming from your car’s engine when driving? If so, it’s possible your vehicle has a rod knock. It generally sounds like a low-pitched thudding noise that varies according to the rate at which you drive. The knocking sound becomes louder and faster as you accelerate.

Don’t Hesitate to Get Your Knocking Car to a Mechanic 

The longer you wait to fix engine knocking, the worse the problem will get. Not only will this mean increased damage and costlier repairs, but you’ll also be putting yourself in danger if you keep driving with a busted engine. 

Now is the time to get your car examined by a professional. 

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