Howling Differential, Advice please!
 

I recently bought a 91 300TE with 4-Matic and 120 K miles. The differential was noisy under load, quiet otherwise. Took it to the leading rear end shop in N. CA (Rearend Specialties) and they diagnosed it as bad pinion bearings. They installed MBZ factory bearings and said it was done properly. Now there is a constant howl at about 40 mph and another at 55-80. Over 45, under 55 it is relatively quiet. Don't know about over 80. Howl is there now with or without load (pressing on gas pedal).

Shop says that this happens sometimes. I'd greatly appreciate some advice as to whether this problem can be fixed and how to approach it with the shop. Hopefully I won't have to resort to the old "sawdust in the grease" trick. Many thanks.

Setting a MB differential is an "art" and very few can accomplish that repair & get correct results. When a ring/pinion have wear patterns & then changed you can expect to hear whining form the diff.

I was afraid that you were going to say that.
It seems to be quiet when first starting out (cold). Because of this, would you think that changing the lube would help? Perhaps a heavier weight oil? I hate to junk the car as otherwise it is great, but this howl is damned annoying.

Try at least 140W weight hypoid grease & see.

What would cause howling when coasting at around 105km/h?

For the record: I tried the 140 weight hypoid grease and it didn't seem to make much difference, although with increased mileage it seems to be quieting down.
Actually, I solved the problem as my elderly neighbor with a bit of a hearing problem, fell in love with the car and bought it from me.

I've never heard of anything so down low & dirty.
Stealing your neighbours hearingaid batteries, just so you can sell him your car . :D