worrisome rear-axle noise
 

did some search on this topic first but have not found a satisfactory resolution ...

i hear a low-frequency sound from the rear axle at low speeds that goes "WOO-WOO-WOO", as opposed to a continuous grinding or whining sound (as i've previously experienced with my differential before).

since my car is a 5spd manual, when i hear the sound peak, i place the car in neutral, and the sound is unchanged, and its frequency is directly proportional to vehicle speed.

from the frequency of the sound and vehicle speed, it seems that more than one "WOO" sound is made per revolution of the wheel. I would say about 2 or 3 per revolution.

I also get a noticable vibration at speed. Jacked up the rears and brought my rears up to speed and the vibration was there. The wheels have recently been balanced though.

could this be made by a bad CV joint? I noticed that the CV joint has 6 internal lobes, which could be producing the 2 or 3 sounds per wheel revolution.

Is the sound on one side or the other or does it sound like it's in the center. If it seems to be in the center it's probably the differential if not it's probably a wheel bearing or even a unevenly wearing tire.

The quick check for a tire issue (which has usually caused this symptom for me) is to swap them front/rear, and see if the noise follows or disappears.

Steve

I would say its the wheel bearing ...

the noise is still there if the rears are jacked up, engine running and in gear.

So it's not the tires.

I hope it's not the diff ...

I've never seen it make that sound before but check the center driveshaft bearing.

I would suspect driveshaft related.

Driveshaft Noise
 

Based on your description, I would suspect the drive shaft center support bearing is dry. I had the same problem on a couple Volvo's I owned and was able to confirm it by injecting some grease into the bearing. I'm not sure if the same test can be done on a Mercedes. I used a grease gun with an injection needle and penetrated the rubber seal on one side of the bearing. If the noise goes away, you have found the problem.

there is in fact a trace of lubricant leaking from the left-side outer CV joint.

but I'm bringing it in tomorrow to the service center, just to make sure.

thanks for the replies, y'all ...