Hello all,

I have a 1980 300SD that whines while the car is moving. It has been that way since I purchased it. When the car is moving, there is a hum, sounds like it comes from the rear wheels but I can not be sure, since I don't have anyone to drive it for me while I listen.

I know it is not transmission related because when I put the car in neutral (while moving) , the noise is still there.

Applying the brake does not make any difference.

The noise is not a grinding noise, but the best way I can describe it is an electric car, electric motor, ect. Makes that humming sound.

After some research on the subject, I see it has also been asked to feel the wheel after a drive, and if its warm or hot, there may be a stuck caliper. I do not feel any of them getting warm or hot.

I tried also jacking the car up and trying to move the wheels by hand, and I have not been able to hear it then.

I am thinking wheel bearings, does that sound like an accurate diagnosis? And if so, is this something that needs to be done right away to avoid other problems?

Thanks and Happy Holidays!

my first car would do this. I figuered it was an old differential. My other two do it a little. When i changed the fluid it was a little quieter

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1981 300SD 512k OM603

I too have the same problem with my 300SD with that humming noise, dose the sound changes when your rear diff oil is changed the huming sound lowers down for a few days and it return back to normal sound, i there are if its not the rear diff 2 possabilities, wheel bearing (as u mentioned) and or the center support bearing...

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Either the wheel bearings or the 3rd member...( differential )..... check the the fluid there first.... but better to just drain and put new in.... be sure to loosed the top fill plug first... IF that is stuck I have posted low tech but high pressure way to loosen it in the archives using the jacking hole in front of the rear wheel and a comealong.... don't want to use big power on that area ... or heat... slow application of power up to the necessary point is the safe way.
At the wheels bearings.... you might could jack up both rear wheels and turn them while a stethescope is placed near the bearings ... or you could just preemptively replace them... and sleep knowing they are in good shape for a long time.

When I got the car I did all the normal fluid and filter changes (oil, ATF, fuel filters, coolant, power steering, and so on).

I consider the differential fluid as being a needed fluid change when buying one of these cars, but I could not get the fill plug loose. Because there were so many other problems I needed to deal with, I put off this task.

Is it harmful to drive the car in this condition, assuming the diff fluid needs replaced, and is causing the noise? Or is it more of those "its ok to drive but sounds annoying" situations?

A whining differential is not a hazard. People have driven tens of thousands of miles on them. Even when you change the fluid, it might not go away. You can try a higher viscosity fluid, like straight 90 to help with the whine.
The only other option is to find a junkyard diff, and hope to God that it doesn't whine also. I don't know of anyone making these or even remanufacturing them.

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A hum indicates something is wrong... I do not think you have isolated it to the 3rd member yet . and driving with the wheel bearings going bad is not a good idea... it could strand you somewhere.... and cause a lot more cost than addressing it now.

Ok, so I FINALLY was able to get the differential fluid replaced with some synthetic. Those bolts were insane. I tried heating the bolts, making a "breaker bar" with pipe, still no luck. The way I was able to get them off is by securing the hex socket, and using my jack up against the hex tool. I got the tires off the ground and everything, all supported by that tool. Still would not come off. Jumped up and down a few times on the rear bumper while the car was in the air and that cracked it loose finally.

I still get the same noise didnt really help at all. Again I dont really care about the noise, but I just dont want anything to get damaged.

One thing to note is that when driving on the freeway with nothing next to me, I dont really hear the noise. But when I am driving next to something like a concrete wall (bridge, getting off on an exit, things like that), the noise is loud.

Other thing is that my car seems to "buck" quite a bit when I let off the fuel, and re apply.....

Dont know if that helps isolate at all, thought I would throw that information in.

Think its still the differential, or could it be something else?

bump

Explain "buck".

Are you getting driveline noises when you left off the pedal? Bad engine mounts, tranny mount, center bearing/bushing, reaf diff mount and sub frame bushings can cause that.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

my opinion is that it is the diff. You could confirm my taking the rear diff cover off and triying to rock the ring gear back and forth. You're trying to see if there is any slop or back lash between the ring gear and the pinion. If there is alot of slop, you might have to get the pinion nut tightened. This can be done without taking the diff apart but if you tighten too much, then you're in trouble. It's best to let a pro do it.

Also see if there is side to side movement of the ring gear. Side to side movement = loose or worn carrier bearings. BTW, the rear of the car should be in the air and the drive shaft should be locked. Also, you'll lose all your diff oil as well, so you may want to just keep driving it as others have said.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

I had that,scared the out of me.One rear whell would get hot.Turned out 25 years of gunk on brake parts.Cleaned pins,sliding areas for pads,Worked pistons in and out with wd.Reassembled,that was 2 years ago.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Thanks for the replies.

rrgrassi, the bucking sound is when I give it "gas" or when I let off the pedal.

I am assuming I should be fine. Like I was previously stating, when I drive next to objects like a concrete wall, the noise reflects and you can hear the "problem" very well. I have tried driving next to large objects going both ways, and the noise is about the same. If it were wheel bearings, the noise should "reflect" on only one side - that or else I have a lot of wheels with bad bearings lol. .

I have changed the diff. fluid and replaced with synthetic. That appears to be all I can or need to do right now, since I dont mind the noise and I would rather not tear apart my rear end. I will leave that to my hot sauce