The noise seems to come from rear.

The tone and loudness are directly proportional to car speed.

It does not seem to be engine noise. I can change engine rev from 2000 to 1500 quickly by releasing gas pedal. But the tone of that noise remains almost the same.

This noise is also present while I am gently braking from high speed. The tone becomes lower with decreasing car speed.

The noise does not change its tone suddenly at tranny gear upshift.

There is no annoying vibration of seat with the noise.

It is somewhat hard to tell tone of noise, but, at about 50 mph, that noise seems to have definitely higher tone than what people usually call 'hum'.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.

If it sounds a bit like a jet engine, than its probably a rear wheel bearing. Does it make that noise with the engine off? Does the noise change if you lightly apply the emergency break? Does the noise change when you turn left or right? At what speed does the noise start and stop?

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Wheel bearing will change on right or left turns, depending on which one is bad.

When was the diff oil changed last?

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I do not know whether the noise comes while the engine is off. The owner's manual says that driving the car at N may damage the transmission. So I did not test that.

I have the noise all the time. I did not notice difference cause by turning left or right, but I will pay attention to that. I did not test the effect of emergency break. I will test it when there is no car behind me.

The noise gradually comes or diminishes at about 20 MPH as I accelerate or decelerate.

Differential oil was changed about one and half month ago. The indi said the color was different from his synthetic. He drained the old oil, filled with synthetic, drained again and then filled again. I do not know when the previous owner replaced diff oil.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.

There was a thread recently on what sounds similar to your thread here. I posted a reponse detailing my also similar observation of the same. The response from those who tried to diagnose the problem didn't really touch right on but only suggested possible causes.

My 87 300D experiences very similar sounds eminating I believe from the rear end and especially between speeds of 50 to 55 mph. I don't hear it any other time however. During these speeds, if I accelerate the humming sound rises in sound volume and increases slightly higher in pitch proportionate to higher rpm. Upon deceleration the humming sound falls in sound volume and decreases in pitch, proportionate to lower rpm.

I have changed differential oil to Mobile One synthetic differential oil, and the humming sound did not stop or change at all. Usually if it is wheel bearings the sound they make when worn can be detected sometimes by turning sharply on a curve in the road, right or left, hearing the sound come on and then off, or any combination of that. I don't get responses like that so I have doubt about wheel bearings, but I might be wrong!

I'll follow this thread and maybe someone out there can figure it out!

How aggressively should I turn left and right? I turned steering wheel left and right so that my car inclines but stays withing lane. I failed to notice sudden disappearance of noise or occurence of new noise.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.

This is only a general rule of thumb! Most automobile rear axle bearings (worn out) will make different types of sounds and even vibration under varied driving and load conditions. All have their own personality. For example, A Volvo 240 wagon I owned recently had a whirling sound and severe vibration at a certain speed from both rear axle bearings. The bearings dried out of grease and with no lube they chattered causing vibration. I greased them and it stopped the vibration.

I don't know the tipical sounds of worn and loose rear axle bearings in a 124 unless of course the sound I'm hearing in my own 124 is that of worn axle bearings. And whether or not they make themselves known by turning heavy right to left, at say 35 mph, and hear the sound either change or disappear for a plit second, is also not known if this telltail showes up on a 124. It does on many cars and I would expect it might on mine, and like yours, there is no change in this sound when I do heavy turns.

And just because the sound didn't change doesn't mean it is not rear axle bearings, it does however indicate the possibility of something else making the sound, so we investigate.

Hopefully someone here may be able to describe what tipical worn 124 rear axle bearings sound like, or maybe bearings inside the differential. I have suspected this to be what I hear in my car, but like you, I'm hoping someone here can help.

So, the hook is baited and we'll wait and see.

How long has your car been making this sound? Is it getting louder, or has it remained the same?

Another vote for wheel bearings. My bearings did what you described consistently, regardless of conditions, turning, etc. You should have seen those things when they came out. I couldn't believe how quiet it was following replacement.

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Chris

'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold

Having no history on my car since before I got it over a year ago, I have no clue if the wheel bearing ever got serviced so I have to go under the assumption they didn't and inspect them and replace if needed, the front too!

Did you replace the bearings yourself? What's involved? Do the bearing need to be pressed on?

Bring your car to an Independent Garage that you trust. Ask him to put the car on a lift and have him listen with his stethoscope (he better have one) to each rear wheel while its spinning at various speeds.

If its a wheel bearing, he should be able to diagnose which one within 3 minutes. Should not cost more than $!0.00. If you are lucky, it may even be free

Don't forget to TIP.


Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I didn't replace them myself, so I'm not sure what's involved. I'm sure someone else can comment.

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Chris

'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold

A mechanic at dealer says the noise is from differential.
The estimate was 4~5 hours labour just to see what is going on in the differential. If the differential needs to be replaced, it will cost well over $1000.
I told the dealer not to do anything for the differential at this time.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.

Well it wouldn't hurt to drain the diff juice and replace with synthetic - total cost about $15 for DIY. It is simply not worth repairing the diff, when you can replace it with a used one for (probably) a lot less $$$. I'd inspect the rear axle shafts closely, make sure the boots are fully intact.

If it's a rear wheel bearing, that's not really a DIY procedure unless you are a very skilled DIY type, pull the rear hub, and have access to a hydraulic press to R&R the bearing. Shop labor is probably 5+ hours, or $500+ total...

FWIW, wheel bearing failures are a LOT more common than diff failures. That is to say I hear of a rear wheel bearing being changed now & then (like Chris's), but almost NEVER hear of a bad diff.

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Dave
Boise, ID
Check out my website photos, documents, and movies!

I have experienced bad differentials and bad rear wheel bearings. A differential will whine. Bearings will have a rougher, less musical note and can be located as someone said, using a stethoscope or a rubber hose (a stethoscope can be too sensitive). The solution is to replace the differential if its that item, using one from a low mileage wreck. I replaced two diffs in my W123's with the lower ratio 2.88 units from '85 cars. I don't know if a lower ratio is available for the '87 300D turbo I often wonder how my car would run with taller gears, I might need to get a pilot's license, right Dave? How about it, MB pilots?!
Dieseldiehard
1971 220 (gas) 4-spd manual 106441 w/ 3.07 300D diff
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280 w/ '85 diff.
1985 300TD 217300
1987 300D 258230

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

I think the 87 300D has about the lowest rear end ratio out there -- 130 mph at roughtly 4600 rpm!

I don't think I will every test the top speed of any of my cars, I certainly don't think I have the reflexes now, if I ever did!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!