5 Reasons Why a Car Makes a Knocking Noise When Driving Over Bumps
If your car is making a knocking noise when driving over bumps, here are 5 components to look at before you bring it in to your mechanic. Before we dive into the diagnosis I need to let you know that not all noises are easy to find, some are hidden in components and are not visible. Struts and steering racks are a great example of hidden noises because the noise is an internal component noise. Video and pictures tell a thousand words and I will be using them throughout this article.
Steering Rack Noise
Steering rack end bushings can wear out and cause a knocking noise. If you suspect your steering rack is causing a knocking noise while driving on uneven road surfaces here is one way to check for play in the steering rack end bushings.
Jack up the front of your vehicle, support it under the frame with jack stands, then grab the front tire at nine and three o'clock, give it a shake from side to side and have someone keep an eye on the inner tie rod, if you notice any unusual movement or you feel a knocking in the tire you may need to peal back the steering rack boot and check for damage, movement, or wear.
A sure sign of a rack end bushing failure is fluid leaking from the steering rack boot located at the inner tie rod. If you take a peak at the boot and you notice it wet with fluid, most likely the rack end seal is leaking because of the play or movement in the rack end bushing.
Broken Sway Bar Links
Broken sway bar links are very common in most vehicles because they take a lot of abuse and they are not very rugged. Sway bar links are small rod type linkage that have a nylon bushing ball socket on each end with a steel ball inserted into it that has a treaded end. The sway bar link is built like a shoulder joint in the human body but with a ball and socket joint on each end.
The fix for this is to replace the sway bar link and hardware, it’s not a very difficult repair unless the sway bar link is in a very tight area like over one of the subframes, if that’s the case it may take a little longer to dig out the broken pieces.
Worn Sway Bar Bushings
Sway bar bushings are not part of the sway bar links, the bushings I’m talking about are located under the brackets that hold the sway bar to the frame or body of the vehicle. The small inexpensive bushings wear out over time and can cause a knocking noise while driving over small cracks and bumps in the pavement.
If you suspect this may be your problem and you would like to do a quick check, have a friend sit in the vehicle and close the door and listen for the noise. The person on the outside of the vehicle will rock the vehicle side to side like you were trying to roll it up on its side. Grab the roof right above the drivers door and start rocking the vehicle, if the person inside the vehicle can hear the noise, your sway bar bushings will need to be replaced.
Replacing sway bar bushings are an easy fix unless they are located on top of a subframe or hidden away, but a lot of the bushings and brackets are easy to get to and only require the removal of 2 bolts on each bracket.
Leaking Struts or Worn Strut Bushings
Worn or damaged struts can make a clunking or thumping noise when riding over bumps and large cracks in the road. Leaking struts are easy to diagnose if the leak is external, but if the leaks are internal you’ll need to do a bounce test to the vehicle to know if the seals are blown internally.
The bounce test is just that, bouncing the car up and down and releasing it to bounce on it own, once you stop bouncing the vehicle it should not bounce more than three time on its own, if it does, you’ll need to replace the struts and I recommend replacing them in pairs.
If you inspect the struts and you notice a lot of fluid and dirt collected on the strut like in the picture, its time to replace the struts, and again, I recommend in pairs. If money is tight you could just replace the leaking strut until you have the money to replace the second one, there is no rule stating that they need to be replace in pairs, I just recommend it for equal ride quality from both sides of the suspension.
Strut bushings are a visual inspection. The strut will have to be removed to inspect the top bushings, but if the bottom of the strut has bushings, they should be easy to locate and inspect. Shock absorbers are inspected the same way, most shock absorbers have a lower and upper bushing and are usually easier to remove than struts.
Worn Ball Joints
There are several ball joints in a vehicle, some located in the front suspension and some in the rear suspension. Ball joints are another ball and socket combination, it’s a nylon socket wit a steel ball. The nylon socket is what usually wears out first and usually its because of the lack of lubrication or abuse (potholes).
There are several types of ball joints and they are not all created equal. Some ball joints are held in by clips, others are pressed in or bolted in, but they all work the same by allowing the suspension to move and turn easily until one becomes worn and noisy.
Not all ball joints are checked in the same manner when testing them for damage or play, some need to be suspended with no pressure, others need to be persuaded with large pry bars, so I recommend either having them checked professionally or check one of the maintenance books or YouTube videos for the proper inspection method for your vehicle.
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