Weird freeway noise 300TD - PeachParts Mercedes-Benz Forum

#1 03-10-2003, 01:03 AM

I would appreciate any ideas about a weird noise I get in my 81 300TD turbo(180K miles) at freeway speeds. It is a single note, always the same, and sounds exactly like the ringing sound you can make with a wine glass by wetting your finger and slowly circling the rim of the glass. It occurs at about 65mph and comes and goes, sometimes much louder. It seems like it might abate at faster speeds -- above 80mph. It will persist for the better part of an hour then go away. It is always exactly the same steady tone. It does not wow, howl or grind. I cannot locate it to the rear or front and the sound fills the car. I noticed it will sometimes halt momentarily if I tap the brakes. Also I noticed that it will sometimes be influenced by applying slight pressure to the steering wheel (i.e., while tracking straight, applying light pressure to one of the steering wheel stops within the sector of steering wheel play). I recently replaced the center drive shaft bearing etc, and recently had front brakes done. I have been having belt screeches lately but no reason to suspect a connection. The car's performance is great, and generally it is in very good condition. Any ideas would be greatly appreciated. I worry about a bearing but wonder also about power steering pump or turbo bearing. The odd thing is that the noise does not vary in pitch with speed changes so it does not appear related to RPM of anything. Thanks

wheelguru · #2 03-10-2003, 05:29 PM

What you are describing sounds like wind noise to me. I had the same thing with a bike rack on my old rabbit. the wheel forks would start to vibrate at hiway speeds but it was only about a 7 mph range. At certain wind angles I would get noise even if the wheel forks were off. have you added anything to the outside of the car lately? notice whether it is in a headwind or tailwind?

#3 03-10-2003, 05:55 PM

Andy: Thanks for the input. I do have an after market high tech ski rack and basket on top but I have had that up there for quite some time and this noise is new. However I did replace my sunroof gaskets last fall (that was a miserable job)and have just started using it again. I will check it for proper sealing. I would be amazed if this noise turned out to be wind because it fills the car and sounds mechanical. Thanks, DW

Bill DeWitt · #4 03-10-2003, 08:38 PM

It just could be the seal in the sun roof...mine was
leaking last fall so I put duct tape on for the winter
and this last Saturday while passing traffic on the
interstate I heard this horrible noise. Pulled off
and check the car and couldn't find any thing wrong
but the noise grew louder and on the second stop
I noticed the leading edge of the tape had pulled
loose for about 4 or 5 inches and when stopped it
would lay back down. So you may have some loose
or broken seal coming out. Bill

The Warden · #5 03-11-2003, 12:33 AM

It could be worse; I have a lumber rack on my truck, along with cab clearance lights. At freeway speeds, especially in a headwind, the wind interfering with the front bar on the rack and the clearance lights (right by each other) makes for a God-awful vibration coming through the ceiling into the cab...I live with it; it's a work truck, but it can be annoying at times

JimSmith · #6 03-11-2003, 07:46 AM

djwhite,

Well, if it is not a wind noise, a few mechanical things to look at are the center shaft support bearing, or the differential. The center shaft support bearing on a 240D is a ball bearing that is sealed "for life" with a coating of a rubber like material, and bonded to a bracket that bolts to the car frame. Once the rubber stuff gets old and cracked the grease leaves and water gets in, and the bearing will start to get noisy. Bearing noise can be very similar to the wine glass tone you hear, as you can excite the rings into a resonant condition. The noise will seem to get louder or fade if you turn your head. You can check this by visually examining the center shaft support bearing, which is on the driveshaft at about the center.

The differential can whine for some of the same reasons. There is rubbing contact where the teeth mesh, and there are ball bearings (usually) in there. IF the oil in the differential is very old, or the level is low, this can lead to wear that can cause noise for similar reasons. If you have no idea when the differential fluid was last changed, it might be a good thing to prepare to do or have done. Sometimes a fill of good quality, clean lubricant can reduce the effects of the rubbing contact (better oil film thickness) and change the resonant characteristics of the parts enough (like changing the level in your wine glass) so the effect does not happen or is less audible when it does.

There are other sources, such as wheel bearings and stuff in the transmission, but from your description, I would start looking at these candidates.

Good luck, and I hope this helps. Jim

#7 03-11-2003, 01:01 PM

Thanks Jim. I replaced the center bearing and mount a few months ago, along with the flex disks , etc. so that is not the problem. Differential is a maybe and I will check that. The strange thing is that the tone when present does not vary except as to loudness. I suppose it is a bearing but the question is why is it intermittent and how to isolate the sound and find it ? Thanks, again, Dennis

leathermang · #8 03-11-2003, 06:10 PM

Maybe the Previous Owner installed a Deer Whistle on your car ?

http://www.xp3hornet.com/

#9 03-11-2003, 08:01 PM

djwhite;

While I agree that the noise that is bothering you may be wind related...............I think that PERHAPS what you are hearing MAY actually be one of your BRAKE PADS lightly contacting a brake rotor.

You stated that you just had your brakes done.

Did you ALSO have any calipers replaced or rebuilt? Just the slightest sticking of the piston in the caliper(s) could cause one of the PADS to lightly rub.

It is also POSSIBLE that one or more of the pads are binding slightly at the top or the bottom. This occurance would have the effect of which you mentioned.

Just food for thought

.

Best wishes always, and keep those diesels humming!

Spo out.