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