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Diagnosing for road noises

March 18, 2015
By Larry DeHays , Fort Myers Beach Bulletin, Fort Myers Beach Observer

One of my new-hire mechanics, many years ago, came back from a road test to identify some sort of noise the customer was experiencing on the highway. He declared that he couldn't hear any noises out of the ordinary and proceeded to his next project. I decided to try to do it myself, since I think I'm pretty good at road-testing cars.

I got in the car and started it up. Immediately, the stereo system practically blew out my ear drums. I shut it off and noticed that the air conditioning was set with the blower on the highest speed, making a loud roar from the dash. I shut it off also, and a short drive disclosed a noisy wheel bearing. The aforementioned mechanic was properly chastised, and he hasn't made that mistake since. Now when he comes back and reports that he can't hear any noises, he quickly follows up with the fact that he had turned off the air conditioning and radio during the drive.

Some road noises are easy to diagnose and some are difficult. A common one is a rumbling sound that develops a higher pitch with higher speeds. This is usually a bad wheel bearing or a cupped tire. They sound similar. The way to tell them apart is to pay attention as you drive over a seam that changes the type of road pavement, from a rough surface to a smooth one, for instance. If the noise changes much, it means it is the tires making the noise. The tread pattern is probably roughed-up, or "cupped," usually from being out of alignment. If the noise seems to stay the same as you cross onto the other pavement, it is probably a wheel bearing. It could be on the front or rear, and on the left or right, and there might be more than one wheel bearing at fault. The car has to be lifted and each wheel checked independently to finish the diagnosis.

I once had a rumbling noise that was very loud inside the car but didn't get a higher pitch with speed. I continued to test it, and finally noticed that it was loudest when going into the wind and quietest going downwind. It turned out to be a vibration between two pieces of wood molding that the owner was carrying on the luggage carrier rack on the roof of the car.

The really difficult noises to diagnose are thumps and rattles that happen on certain bumps or driving maneuvers. It's hard to get a mechanic to ride along on his creeper under the car while the driver hits some speed bumps to duplicate the noise for him. (Just kidding). If we can't see something moving when the thump happens, we can't be sure what is making the noise. We can raise the car and look things over, but sometimes a slightly loose suspension part will not show itself unless you hit a hard bump, and then you can't be looking under the car.

One of the first things we need to know about your unwanted noise is: what type of driving is going on when it happens. Will it happen sitting still? If so, we don't need to take it for a road test. If it will happen while sitting still it is much easier to trouble-shoot. That kind of rumbling noise, for instance, might be a water pump bearing, or some other bearing under the hood.

I once had a car with a thumping noise whenever the driver accelerated or braked. We checked all of the motor mounts and transmission mounts, axle struts, sway bar bushings and everything else we could think of, and finally found a loose bowling ball in the trunk. Sometimes this is a funny job.

Larry DeHays is the author of the book "The Car Care World," a compilation of his most popular columns. It is available now through Amazon, Barnes and Noble, or at the DeHays Automotive office at 17617 Broadway Ave. He has been an ASE Certified Technician for 37 years and an arbitrator for the Florida Lemon Law for 16 years. For more information go to or .



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