My 240D has an increase in noise above 50-55 mph. It sounds like a "boom" or resonance. It may be related to the exhaust system. However, my first guess is that it may be noise from the cooling fan as it is not driven thru a clutch and runs at full engine rpm. Has anyone replaced the original fan with one of the aluminun fans typically used in hot rod applications? These fans have a very aggressive pitch to move a lot of air at slow engine speed, but the blades tend to flatted out as speed increases, moving less air and creating less noise.
I also have a vibration which seems to be in the 50-60 mph range, and decreases above about 60. It seems to be the rear wheels as I feel it in my seat and not in the steering wheel. However, whem I jacked up each rear wheel in turn and ran it up to 30 and then 60 mph indicated on the speedometer--there was no vibration. (I used 30mph as I believe that when you jack up one wheel, it spins at twice the indicated speed--but just to be sure I also checked at 60mph indicated) I guess it may still be an out of round tire, or even a front tire. I will check those possibilities a a day or two, but was wondering what other ideas some of you might have.
Thanks
Michael

Michael, I had a similar vibration which turned out to be a flex disc on my 240D. Check your repair manual , there's one on each end of the drive shaft to absorb vibration , but if worn or cracked you will feel it , most likely through the tranny or what you described. $40 part, so-so installation, couple hours. Big Ron

If it were just the fan wouldn't it do this in every gear then? Yes I've seen at least one of those upgrade fans on a w123, never really thought about it before though. Oooh, flex disc, better make sure that one doesn't break! ($$$$)

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

One other thought, have you checked the CV joint boots?

I had something that sounds a bit similar, although it sounded more like a knocking...and also, although it always started at freeway speeds, once it started, it would make noise 'til I stopped. So, that may not be it, but it's something to check out.

Hope that helps some...good luck!

Warden,
The CV boots show some cracking, but none are all the way thru; they hold the lubricant. They are on my "list" but not urgent until they start to leak.
As for the drive shaft flex couplings--wouldn't that also show up when i jacked up the wheels and spun the tires at road spped? Or, are they something that needs to be under a load to show the problem?

I would imagine they would show up in that test, but just to be safe it is worth glancing at them. If they are cracking, and you can see the thread or whatever it is inside of them you need to replace them, they can cause catastrophic damage to your driveline if they fail. There are two flex discs, one in front, one in back.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I have a similar noise on my 300d. it has been there since I got it. Is yours rythmic at about 1 second intervals? mine changes interval if I let off the gas pedal - it will usually slow down to a longer interval, but the pitch does not change.
It is not flex discs for me, I already changed them. I have a new transmission mount and one new motor mount (long story). Next I am going to change the differential fluid on general principle, and then the center bearing, as soon as I crawl back underneath and figure out where it is.

__________________
andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles

I have a droning very much like that described by wheelguru in my 1982 300D. It started precisely when I changed the rubber transmission mount.

I've given serious thought to putting the old mount back on the car as a trial.

Ken300D

Oh, I forgot to mention, it is not the engine or transmission (i don't think anyway), those were swapped as one piece 1 1/2 years ago. the noise was there before and after. -Andy

__________________
andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles

I MAY have found the problem. There is a support for the exhaust that is bolted to the rear of the transmission mount. Mine was incomplete--I just had the arm/ cradle that holds the exhaust pipe--but no clamp. I fitted a clamp today, and will test drive tomorrow. The pipe appears to have been laying in the cradle, but with no clamp to hold it tightly in place it was able to move and buzz-- at least thats what I think now. I'll post back after I drive tomorrow.

Yeah, that was it! I drove 200 miles today, and the noise is gone!