Knocking Sound Under Car When Accelerating – Causes & Fixes

When you press down the accelerator you expect your car to go forward, what you don’t expect is to hear a knocking sound. But what does it mean when you hear a knocking sound when you hit the accelerator?

We’ve highlighted ten potential causes for you here, and we’ve even brought up a few basic troubleshooting tips to help you narrow it down and figure out what’s happening.

Don’t settle for or ignore a knocking sound from underneath your vehicle because it’s more likely to get worse the longer you ignore it.

10 Causes of Knocking Sound Under Car When Accelerating

Below we’ve highlighted ten potential causes of a knocking sound under your car when you’re accelerating. Our first two suggestions apply if the knocking sound is coming from directly under the middle of your car, while the other eight suggestions would create a knocking sound near your engine.

1. Zip-Tied Driveshaft

It’s one of the oldest tricks in the book, and it still makes plenty of people’s hearts skip a beat when it happens. This isn’t something that happens by itself, but it’s a pretty common prank.

Essentially people put a zip tie around the driveshaft. The zip tie doesn’t do any damage, but when the driveshaft spins the long end of the zip tie hits against the vehicle and makes a loud thwacking sound. Do yourself a favor and give the driveshaft a quick peek before taking your car to a repair shop.

2. Damaged Driveshaft Components

The only component that should move directly under your vehicle when you’re accelerating is the driveshaft. So, if that’s where the sound is coming from the problem is something with your driveshaft.

But typically the driveshaft itself stays in pretty good shape since it’s pretty much just a long metal tube. However, if you drive off-road a lot, something can come up and hit it, and sometimes the bolts and u-joints holding the driveshaft in place can start to develop problems.

3. Faulty Spark Plugs

Spark plugs wear out, and as they do, they don’t create the necessary spark to keep the engine running smoothly. While it might not make sense that a lack of spark creates a knocking sound, it all has to do with the overall timing of the engine.

If one cylinder isn’t firing when it should, or not at all, it throws off the timing for the entire engine just a bit. That means when those spark plugs do ignite, it’s just a fraction off the right time. This creates a little bit of a misfire in those cylinders, and those misfires create an engine knock.

Spark plugs can last between 60,000 and 100,000+ miles, but eventually, they will start to fail. If you want to learn more, then here’s an article where we cover the most common symptoms of bad spark plugs.

4. Wrong/Low Octane Fuel

While many vehicles run just fine on lower-octane fuel, but if you have a vehicle that specifically calls for higher-octane fuel, then putting in fuel with a lower-octane rating can create problems. One of the most common problems is an engine knock as the engine struggles to perform with the lower quality fuel.

While the engine might survive one or two tanks of lower-octane fuel, it’s only a matter of time until it leads to engine damage. We don’t recommend running lower-octane fuel in your tank, and if it’s already there we recommend draining it.

Another option you have is to dilute it with the right octane fuel, but this still isn’t the ideal method and it still has the possibility of creating problems until the engine can get through the lower-octane fuel.

5. Defective Fuel Injectors

Fuel injectors are the final stop for the fuel before it reaches the combustion chamber, and without the right amount of fuel in the combustion chamber it’ll throw off the timing the same way a faulty spark plug would.

It’s important to note that the fuel injector is one of the only fuel system components that will affect only a single cylinder. Other fuel problems will affect all the cylinders, and in these cases, the engine usually won’t start.

Fuel injectors typically last between 50,000 and 100,000 miles, with higher-end fuel injectors on modern vehicles typically making it well past the 100,000-mile mark.

6. Faulty Timing/Timing Belt

When everything inside your vehicle’s engine works the way it should it’s in perfect harmony. But this harmony all revolves around everything having the right timing. Each part needs to do its job at the right time, if not an engine knock can lead to a drop in performance.

Often the engine timing is off because of a problem with the timing belt. Timing belts and chains typically last about 100,000 miles, and if your vehicle has more miles than that and an engine knock, there’s a chance it’s time to replace the timing belt on your vehicle.

7. Improper Air/Fuel Mixture

While this is certainly a reason your vehicle might have an engine knock when accelerating, figuring out the exact cause can be a little more complicated. There are a ton of different things that can throw off the air/fuel mixture, but typically there will be a check engine light that helps point you in the right direction.

The problem could be a failing fuel pump, clogged fuel lines, excessive carbon build-up around the intake, a stuck throttle, or faulty sensors along the intake system. Look for a check engine light and then go from there with your troubleshooting.

8. Worn Rod Bearings

This isn’t an issue you typically have to worry about if you keep up with all the routine maintenance. But if you’re simply unlucky or you didn’t keep up with the routine maintenance the rod bearings inside the engine can start to wear out.

When that happens the excessive play will create issues and an engine knock. But if you keep up with all the routine maintenance on your engine it’s not uncommon for rod bearings to last at least 500,000 miles.

9. Faulty Knock Sensor

Did you know that most modern engines have a sensor that detects minor engine knocking and adjusts the timing to fix the problem? It’s a great feature that helps protect the engine, but if the knock sensor fails then the engine can’t adjust to fix the knock.

Knock sensors typically last a long time, but they can fail. The good news if your engine has a faulty knock sensor is that compared to most options on our list, it’s a relatively inexpensive fix. But if your engine has a faulty knock sensor, it should also have a check engine light.

10. Damaged Belt Tensioner/Pulley

Sometimes you think the engine knock is coming from inside the engine when in truth it’s coming from outside the engine. This type of engine knock will sound a little bit different, but if you don’t know the difference between the two sounds already it’s easy to mistake the two.

Take the time to rule out an external engine knock before diving too far into the troubleshooting process. If there is an external engine knock, it’s most likely coming from a damaged or loose belt tensioner or pulley on the front of the engine, you just need to figure out which one!

Basic Troubleshooting Tips

If you notice a sound coming from underneath your vehicle when accelerating, there are a few basic troubleshooting tips you can use to help you narrow down the problem. First, see if you can get the problem to happen when the vehicle isn’t moving.

Put the car in park, and then press down the pedal. If the knocking sound is there, try to pinpoint the exact location. If the car is in park, you can rule out anything with the driveshaft, and the problem should be coming from the engine bay.

Next, see if there’s a check engine light. If there is, figure out what it’s for and go from there. Finally, use an automotive scan tool to look at the various sensor readings. You’re looking for anything out of place to help point you in the right direction since there are a lot of potential causes.

If you can’t figure it out, there’s nothing wrong with taking your vehicle to a professional mechanic to have them figure out what’s going on for you.

Final Thoughts

Any time your vehicle starts making new noises, you ought to take notice. Sometimes it’s something small and insignificant, but if it’s a knocking sound coming from underneath your vehicle, that’s likely not the case.

Now that you know some of the common causes and how you can figure out what’s going on, it’s time to head out and take a look at your vehicle. If you can’t narrow it down, go ahead and take your vehicle to a professional mechanic so they can tell you exactly what’s going on.

The sooner you catch the problem the more likely you can keep it from turning into something big and even more expensive!

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