Troubleshooting The Knocking Sound When Accelerating At Low Speed. What’s The Cause?


As a driver, you’ll often find that many car problems make themselves known by the sound that they make. For instance, a knocking sound that occurs while accelerating at low speeds is a clear sign of a problem. However, that sound could point to several issues, some more severe than others.

Firstly, a knocking sound that happens while slowly accelerating could be harmless. For instance, you might have loose cargo inside your trunk or something rolling underneath your seat inside the car.Alternatively, a component might have become dislodged underneath the vehicle, like the exhaust. Knocking can also be from old spark plugs or damaged suspension components.

Identifying the root cause of that knocking is crucial, so you can troubleshoot and solve the problem quickly. The article below will show you the most likely reasons and what solutions you can use to resolve the issue.

Why Is My Car Knocking When Slowly Accelerating?

A knocking sound from your car as it accelerates will cause anyone stress, even seasoned drivers. However, there’s no need to panic.

As you’ll see below, some of the likely causes aren’t mechanical at all. That means it could be a harmless object moving around too much somewhere in the vehicle.

However, there’s a possibility that the source of that noise could be a mechanical issue that requires your immediate attention.

Below, you’ll discover a handful of different reasons your car is knocking when slowly accelerating and what you can do to solve each of them quickly:

Read: How To Evacuate Car AC Without A Pump?

1. A Dislodged Component Under The Car

Suppose you hear the knocking sound coming from directly underneath your vehicle as you begin to accelerate. In that case, the most likely source of that noise is a dislodged component from the car’s underside.

This root cause is also more likely if you’ve recently gone over a bump or accidentally run over a curb. Doing so likely caused the ground to hit something underneath the car and cause it to come free.

Some common components that could come loose under your car include the heat shield, exhaust, or underlayment.

As you accelerate at low speed, those loose components will shake and hit your car’s body, causing the knocking sound you’re hearing.

How to fix it: Firstly, it’s best to avoid driving for the time being. That’s because the more you drive, the more damage you’ll cause to that component.

Whether you do it at home or at your preferred workshop, you must secure the component to your car’s underside. That could mean replacing the screws, mounts, or adhesives as needed.

Once the part is secure, the knocking sound will stop.

Read: What Is A Link Stabilizer On A Car, And What Does It Do? (With Pictures)

2. Loose Cargo In The Trunk Or Under Seats

Another reason you hear a knocking sound while accelerating is that there’s loose cargo in your trunk or inside the cabin, under the seats.

Remember: as you accelerate the vehicle at low speed, the vehicle’s weight will shift towards the back. That motion and inertia will cause loose cargo to move around and hit metal parts in your car, generating that knocking sound.

Suppose the knocking sound comes from your trunk. In that case, the spare tire or your tools might be loose. They’ll hit each other or parts of the trunk and make a knocking sound.

Besides that, a metal object like a drink can or bottle could roll underneath your seat and hit its mounts, bolts, or even the car’s floor. That would also cause the same noise to occur as you begin accelerating.

In this case, the noise source is harmless and poses no real problem.

How to fix it: Firstly, you must look around your vehicle to locate any loose cargo inside the trunk or under your seats. You can then remove anything that doesn’t belong in the car or provide it with a safe storage space.

If the root cause is a loose spare tire, then you must secure its mount and any other tools you might have.

Read: Fuel Not Getting From Carb To Engine? What To Check And How To Troubleshoot It?

3. Old Spark Plugs

Unfortunately, the knocking sound you hear when accelerating at low speed could result from a mechanical or technical problem. For example, the knocking could come directly from the engine, caused by worn-out spark plugs.

Good spark plugs will cause each engine chamber to combust at the correct times without any problems and in perfect synchronicity with the others.

However, worn-out spark plugs will not deliver the spark necessary to combust the fuel in the engine. So, instead of combusting, the mixture detonates at the wrong time, causing the knocking sound you hear. 

How to fix it: Ideally, you should replace worn-out spark plugs with new ones as soon as you hear knocking from the engine. 

However, as a short-term fix, you can scrub those plugs clean with a wire brush to restore some of their functionality.

Read: How To Identify What Engine My Car Has?

4. Damaged Suspension Components

Lastly, you might have damaged suspension components if you hear the knocking sound coming from one of your wheels.

The suspension system plays the crucial role of keeping your wheels planted firmly on the ground. It also minimizes vibrations and noise to make your driving experience a more pleasant one.

The system itself consists of several parts attached to each wheel. For example, each wheel has a control arm, various bearings, a steering knuckle, and so on.

Given how these parts are directly above the ground, they’re pretty exposed to damage from road debris like rocks. Besides that, any hard impact against the wheel, like hitting a curb or an exceptionally deep pothole, can quickly damage those suspension parts.

More specifically, the knocking sound you hear when accelerating at low speeds could be due to a suspension component that has come loose or that has broken off entirely.

Suspension knocking sounds can come from any of the four wheels, so you’ll have to pay closer attention to narrow it down to its source. The knocking might also be accompanied by other sounds like grinding or squealing, depending on which part is affected.

How to fix it: You’ll likely have toreplace one or more of the affected suspension parts. However, you should first perform a detailed inspection of the damaged section to be sure.

Sometimes, all that’s needed is to reattach a part that has come loose or was detached due to the damage. Doing that could be enough to solve the problem and save you plenty of time and money.

However, for any parts that can’t be fixed, you’ll need to get new ones to take their place.

The suspension system isn’t just crucial for maximizing comfort and minimizing noise. When there’s a damaged part, that could place added stress on other suspension components, causing them to wear out prematurely.

Read: What Are The Signs Of An Overcharged Car Battery?

Final Thoughts

Firstly, never ignore a knocking sound that occurs while accelerating your vehicle. The noise could be from a harmless source, or it could suggest there’s a severe problem that requires immediate troubleshooting.

On the relatively harmless side of things, the noise might be coming from loose cargo inside the trunk or under your seat. Besides that, a component might have become dislodged under the vehicle and need to be reattached. However, knocking can also come from the engine due to worn-out spark plugs or from one of the wheels due to a damaged suspension component.

Leave a Comment