I just bought a 1999 C280 elegance with about 140 000 km on the clock. The car is in an excellent condition and drives like a dream, but noticed something that really bothers me.

At a speed of about 70 km/h I hear a whining noise. Almost sounds like these old sirenes when they used to warn a city of an air attack (not that I really heard these - as seen on tv ). At first I thought it came from inside the car, like the radio or something, but when I open the window one can actually hear it come from the outside. The sound is quite noticable, but not deafening. When I increase or decrease the speed to above 80 or below 60, the noise goes away.

Does anyone know what the source of this sound is?