Knocking Noise Under My Car When Driving: What Could It Be?

Regular car maintenance is a given but we don’t think about all of the working parts that keep our cars running until something starts acting up. One noise is a sure sign that something is wrong: knocking. Whether it happens when you’re idling, driving, or accelerating, a knocking noise is never a good sign. A knocking noise can have a myriad of causes. We are going to explore a few.

Quick Answer: What Could It Be?

A knocking noise under your car is something that makes drivers cringe when they hear it. Nobody has time for car trouble. Knocking can be caused by any of the following things.

• Bad Tires
• Wheel Bearings
• Ball Joints/Lower Ball Joints
• Loose Lug Nuts
• Belt
• Axles
• Suspension
• Struts/Strut Mounts
• Stabilizer Bar/Right and Left Control Arm

It is important to keep your car in a safe operating condition. This is accomplished by routine maintenance and addressing problems as soon as they arise. One problem you could encounter is a knocking noise. Diagnosing the problem is the first step in repairing it.

9 Possible Causes for the Knocking Under Your Car

Knocking noises under your car could be caused by various culprits. Some are easy to find and fix, others may require the eye and skill of an experienced mechanic. However serious the cause, it is important to address it as soon as possible to ensure a safe ride.

Bad Tires or Uneven Tread

A knocking sound coming from your tires can be caused by bad tires or uneven wear.

Regular tire maintenance is important: Tires require regular maintenance and should be rotated every 3,000-5,000 miles and rebalanced every 12,000 miles. Tires need the correct air pressure which varies from vehicle to vehicle. Well maintained tires can last from 25,000 miles to 50,000 miles.

Tires can wear down faster based on driving habits: Tire tread wears down naturally over time but wears down faster if proper care is not taken. Improper tire inflation, misaligned tires, spinning your tires, hard braking and fast acceleration all cause your tires to wear down faster than normal.

What to do if your tires are bad: If you notice uneven wear, you can have your tires rotated. If they are bald, meaning tread depth has reached 2/32, they should be replaced.

Bad or worn out wheel bearings

A noise that gets louder as your vehicle accelerates could be bad wheel bearings

What are wheel bearings: Wheel bearings are a set of metal balls that are held together by a metal ring. The metal balls roll between two rings of metal allowing it to freely rotate.

What to do when wheel bearings go bad: When one goes bad, there is more friction as the wheel turns and the tire can begin to jiggle. A worn out wheel bearing can cause the wheel to lock up while driving.  Bad wheel bearings should be replaced as soon as possible by you or a mechanic.

Bad Ball Joints/Lower Ball Joints:

A clunking noise, extra vibration and uneven steering could be caused by bad ball joints.

Where are the ball joints located: Ball joints are a part of the front suspension of your car. They are a ball and socket system. Due to supporting the weight of the vehicle, the low ball joints which are bigger, usually wear out faster.

Replacing your ball joints: When ball joints are worn out, your front suspension can get loose and can result in a clunking noise. If the ball joint completely fails, the wheel can come off of the car. Replacing your ball joints requires a ball joint press.

Loose or Damaged Lug Nuts:

A light knocking or rattling can be caused by loose or damaged lug nuts.

Lug nuts are on every wheel of your vehicle: Lug nuts are used to secure a wheel to the hub. A wheel can have anywhere from four to six lug nuts per wheel. Lug nuts do not typically wear out and require no regular maintenance.

How to deal with a damaged lug nut: Lug nuts can be damaged by improper installation or over tightening. When a lug nut is loose/damaged, it can create a light knocking or rattling sound. If one is loose, tighten it and if one is damaged, you can likely find a replacement at your local auto store.

Broken Serpentine or Timing belt

A knocking or slapping noise under the hood could be caused by a loose serpentine or timing belt.

Identifying the two belts: A serpentine belt is flat on one side and grooved on the other. A timing belt is flat on one side and has notches or knobs on the other.

Belts generally have a long life: A serpentine belt generally lasts up to five years or 50,000 miles. A timing belt will usually last from 6-10 years or 60,000-100,000 miles.

What to do when one goes bad: Either belt can cause a knocking noise under the hood if it is loose or damaged. Both belts can be replaced but the timing belt is much more expensive and complicated to repair.

Worn Out Front or Back Axle

A knocking sound that gets louder with sharp turns may be a worn out axle or CV (constant velocity) joints.

How many axles does your car have: Most cars have two axles. A general rule is one axle per set of tires. Larger vehicles with more wheels have more axles.

Signs your axle is going bad: You may hear a knocking sound when turning or notice grease on the tire edge.

Replacing a bad front or rear CV axle: You can drive on a bad CV joint for about five months but it should be replaced before it causes damage to other parts of your vehicle. A bent axle cannot be repaired and must be replaced.

Worn Out or Damaged Suspension

A knocking or rattling noise while driving on a rough road could be a sign of worn out or broken suspension.

Components of your suspension: The suspension system consists of shocks and struts, springs, control arms, bushings and ball joints.

Signs of a worn out suspension system: The suspension system serves to ensure a smooth ride. If you are experiencing pulling to one side, difficulty steering, noise while turning, or knocking or rattling while on a rough road, some components of your suspension might need attention.

Repairing parts of your suspension: The good news is, even though it is a complex system, it is possible to replace only the damaged part of the system.

Bad or Worn Out Struts or Strut Mounts

Unusual noises coming from the suspension system, or knocking when going over bumps could be a sign of bad struts or strut mounts.

Struts and strut mounts have an important purpose: Struts and strut mounts are the part of the suspension system that provide support to the suspension springs.

Signs your strut or strut mounts have reached the end of the road: Bad struts or strut mounts can lead to uneven tire wear and expedited deterioration of shock absorbers. One sign they are going bad is unusual noises from the suspension system or knocking when driving over bumps.

What to do when they are beyond repair: It is recommended that struts and strut mounts be replaced in pairs since they typically endure the same amount of strain.

Worn Out or Broken Stabilizer Bar or Links

A knocking noise while going around a corner or over a bump could be a sign of a broken stabilizer bar.

What are the stabilizer bar and links: The stabilizer bar, AKA sway bar is a U-shaped bar that connects to the front wheels of a vehicle and shifts the weight while turning to prevent rollover. The links connect the stabilizer bar to the suspension system.

Signs the bar or links are wearing down: If the stabilizer bar links are wearing down, you may hear a knocking, clunking, or metal-on-metal noise especially while turning a corner or going over a bump.

Repairing or replacement: If one sway bar link is worn out, it is best to replace both at the same time since they receive the same amount of wear. You can replace a broken sway bar without replacing the links if they are in good condition.

Wrap Up

You now have a good idea of what could be causing the knocking noise under your car. If you are unable to identify the source on your own, employ a mechanic to diagnose your problem as soon as you can.