I recently got my 300CD out of being parked in my garage, and took it for a drive. I noticed that it runs smooth and quiet, idles nice, etc.

But when driving on the road, at ~40-50, I get a whine or moan sound from under the car.

It doesnt come on gradually that I can tell, rather it comes on abruptly at the 40 or so point, and then is not noticeable (dont think its merely due to engine/road noise) around 50.

Letting off of the throttle makes the sound go away, so it may be related to the drivetrain. More throttle doesnt change it, it just continues on until I go a bit faster and then it disappears.

My thought is that perhaps from all the sitting, whatever minor oscilatory/vibratory modes that may exist for the driveshaft, perhaps has a slight motion and perhaps there's a lack of lubrication in there.

My gut is that it is NOT the diff, because Ive had a bad, whiny RWD diff before and the location and sound differs. Also, the CV axles are OE Mercedes, installed before I parked the car in my garage. So Im pretty positive theyre OK.

Any suggestions?

Thanks!

Any thoughts on what it might be or how to best test?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Perhaps the center support and or bearing or u-joint got tight from sitting. Look at the u-joint cup/seals extra close for any sign of rust.

Does the car sag in the rear at all? Wondering about a collapsed diff mount which may be made from unobtainum IIRC


Sounds silly but what does it cost? Back it up quite some distance and then do slow full turn figure eights forwards and backwards. It may free something.

Agree probably not diff from sitting.
Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

IIRC it’s the TD diff mount that is different somehow...

The car has always had slight sag to my eye, but it’s not definite that it’s truly the case...

You would t see it today when I took this pic:


Good idea with the backwards driving! First ai need to fix a stuck caliper!

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Speedometer cable?

In your other thread you show a serious over heating problem on one of the brakes - I'd fix that first and then see if there's still some weird **** going on at 40 - 50 mph

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

As suggested take care of the caliper first. A sticking overheated brake with a slightly warped rotor may be the problem, especially if it is a rear caliper.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

That did give an atrocious scream once in cold weather when I first had the car... changed. Don’t think that’s it... though routing the new cable was a challenge to avoid bouncing and whatnot.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Yes I will be taking care of the caliper first. Will do it as I have free time, which isn’t very soon/often...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

To follow up, drained the fluid. There were no metallic chunks or shards, but there was a good deal of metallic sludge in there.

The grit in the pan here was from me not cleaning it well last time.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Temporarily put some 75W-140 gear oil in it the Diff to see if it quiets it down.

__________________
2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)