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Zoonhollis 10-23-2003 01:06 PM

Harmonic Drivetrain Noise
Okay, I've used the search tool and found some good info, but nothing that describes my problem exactly.

I think I've eliminated the wheels/tires as culprits for this strange highway noise I'm getting at speed. The car is an '84 300D with 230K miles. I'm pretty sure it's something in the driveline--perhaps a driveshaft out of balance. The sound is like:

wwwWWWOOOWWWwww...wwwWWWOOOWWWwww (ad nauseum) other words, it's not constant, but fluctuates harmonically. Could it be worn flex discs? A fatigued tranny mount (deviating the angle of the shaft)? Out-of-balance driveshaft?

I've managed to get this car fairly quiet at highway speeds, and now this noise is particularly apparent.

JimSmith 10-24-2003 08:03 PM


Noise sources rich in harmonics like you describe are usually items with ball bearings. So, the first place I would look is the driveshaft center support bearing. This item is a radial ball bearing with the outer race bonded to a rubber donut, that is then bonded on its outside diameter to what looks like half a pipe hanger for a 4 or 5 inch diameter pipe. When the rubber breaks down water and grit get into the bearing and the isolation the rubber donut provides diminishes and you have a "sound short" to the car body. The body selectively responds to its favorite frequencies so you hear the whole system interacting with the source of the stimulus, the drive shaft turning.

And then, again, it can be the bearings anywhere in the rest of the drivetrain and the front wheels. I would start with the drive shaft center support bearing though as I have had this go out a few times on different cars and it sounds much like your visual description of the noise (pretty good depiction of a noise).

Good luck, Jim

Zoonhollis 10-25-2003 07:30 PM

As usual, thanks for the reply and the good info, Jim.

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