W123 300D Sedan: Strange Drone/ Humming Sound (Vid Included)
 

Greetings!

Drove around the Cotswold area in the UK today. Lots of curvy roads that can be tackled at speeds between 40-50 mph.

Today, the 300D's (133K Miles) rear end began making strange noises. Aside from a rodent like squeaking every now and then, it made a recurring loud humming sound. Video of the sound attached:

https://www.youtube.com/watch?v=BJWTtjv9UA0&feature=em-upload_owner

Here are the symptoms:

1) Sound occurred only when turning slightly in one direction right at speed
2) Changing direction caused the sound to stop immediately
3) Braking lightly also caused the sound to stop immediately.
4) The car didn't drag or decelerate abnormally

Any experience with this problem is welcome...and Thanks!!

Rear axle joint. Have you checked the swing axle boots? One or more may be split open resulting in a lack of lube for the swing axle.

Axle boots appear fine. No cracks and no leakage of grease.

It might still be an axle problem. How many miles are on your vehicle?

On mine, a W126, I found a worn out CV joint. There wasn't any leaking or cracked boots. The only thing noticeable was a rumble that I originally mistook for a driveshaft problem.

However, when I raised the rear and left the suspension hanging, I noticed a strange offset, or s-curve, in the worn joint. It was very evident when I removed the axle!! I'm glad that it didn't come apart while on the highway!

133K miles total. I can definitely explore this further.

Similar sound from a worn differential pinion bearing in my 300CD. Difficult to repair here in the colonies since it needs a proprietary case spreader to install replacement bearing.

Replaced entire differential with a used and got lucky.

How would gently braking (but remaining at speed) stop the sound from a differential?

...


Rear wheel bearing...

Opposite side from side turned when it makes the noise.

Not an easy task... easier to swap the trailing arm... any 123/126 will work.