Date registered: Aug 2005
Vehicle: 1972 250C, Ducati ST
Location: Norfolk, Virginia
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
RE: 88 300 SE Rear Humming Noise
Does it do it with the car completely stopped? If it does, you can probably rule out any kind of suspension/drivetrain/brake issues.
If it only does it with the car moving, you might have a worn wheel bearing, worn differential, or dragging brake. I've had that identical noise on several cars, and one of those was always the problem. The vibration from the movement of the worn/dragging part would set up a resonance in the trunk cavity, causing a droning/humming/low-pitched whistling sound. At higher speeds, the frequency of the noise would be too high to make the trunk cavity resonate, and the sound would go away.
Try driving through an empty parking garage to use the echoes to amplify the sound, and while you're at it, try driving slowly with the trunk open to see if the noise gets louder/quieter/goes away.
After a drive, hold the back of your hand near the face of each of your rear wheels (remove the hubcap if you have one). If one side is noticeably hotter than the other side, it's probably a dragging brake. If not, it's probably the wheel bearing. Mercedes differentials are pretty sturdy, and you're only hearing the noise from one side, so that's less-likely to be the problem. Even so, if both rear brakes and both rear wheel bearings check out fine, drain your differential oil and look for metal shavings just to be sure.