New noise ,a honing sound from the rearend when let off the gas pedal
 

A honing sound thats coming from the rearend of my 83 300cd .It started just a couple of days ago ,I put new half shafts on 5 months ago and it doesnt sound like that is the issue.

slack whine from rear is often the diff, but it could easily be the trailing arm bearings... does the noise change pitch on tight left/right turns?

My bet is ring/pinion wear causing noise... you could dump the fluid, and fill with 75w140 synthetic and see if the sound goes away...

Ill try that ,thanks.She is a very old girl with 400k and climbing,Ive just recently taken it off the road to service the frt end ,verysloppey,and did a quick jaunt to the parts store and noticed the new noise.

Someone has to ask. What company's Half Shafts did you use?

Advanced auto parts supplied the shafts ,name ,not sure .

price? the AAP bargain axles have the longevity of ice on the sun...

When He used that were New that set of a tiny alarm and that is why I ask the question.

Next I am wondering if He has the Annular Axles that have the Bolts that come loose?

In any event I don't think I would rule the Axles out as a cause of the problem.

I think the New OEM GKN Axles are the only New aftermarket CV Axles that anyone could trust at someting like $399-$450 each and Pelican sells the Genuine Mercedes CV Axles.

More specifically, the rear pinion bearing set. The pinion is forced forward (putting added pressure on the rear bearing) when the wheels are driving the driveshaft.

Pinion bearing
 

I had identical noise from my 83 300CD. Repllacing the pinion bering is a difficult job requiring a case spreader. I could not do it nor could I find a local independent that had the tool. My local Benz dealer did not either, or maybe used it as an excuse to not work on the old car.

My solution was to replace the differential with a known good used one. Silence rules! Anyone with a spreader want to buy the old one?

BTW, that noise is non-critical so long as it does not get very loud over time or louder suddenly. Mine had a sort of soft hiss when unloaded and I went about 30k miles before the repair.

Indeed a classic sound caused by =>

At the risk of being pedantic - the spreader tool is used to set the pre-load on the axial bearings by getting the casing to flex outwards and then clamp up against the (axial) bearings. With correct shimming the crown wheel should be in such a position that the backlash between the crown wheel and the pinion wheel is correct.

Whining / droning / honing sounds are caused by poor contact between the crown wheel and the pinion wheel. In the event that the clearance of bearings on the pinion wheel (set orthogonal to the axial or side bearings - in a longitudinal sense) is too great then the pinion shaft moves as described above. The contact patch position changes and you get the sound.

In my somewhat limited experience you can dismantle the differential and crown wheel assembly quite easily from the casing without the spreader tool. Replacing the pinion bearings is then possible with the help of a hydraulic press; setting up a correct crown wheel to pinion wheel contact patch is the tricky thing. I'm in the process of doing this at the moment on a W201 differential.

http://www.peachparts.com/shopforum/diesel-discussion/344307-w201-differential-refresh-thread.html

This is, however, yet another project on the go - I'm going to have to make my own spreader tool to get the axial bearing pre-load correct...

Cheap fix => a change of differential oil might help for a while. (Its not like you've got a crappy W201 differential that probably came out the factory whining - the W123 ones are better lasting)

My dad's previous 300SD had a diff like that, tons of slack in it and a rumbling/whining when coasting. In response to the above comment, my 2.85:1 diff in my 190 is original and silent with 208k on it....virtually no slack in it at all. It all depends on how well it was maintained.

Actually, the reverse happens. Under load, the pinion is forced away from the ring gear, that's why the bearing closest to the pinion is larger. Under decel, the pinion tends to want to move toward the ring gear. Checking the preload on the pinion bearings and a possible tightening of the retaining nut may cure the problem.

You are right. Looks like my axial dyslexia problem has resurfaced.