I recently replaced the rear pads, rotors and one caliper on my '90 300e. Since then I have noticed that at highway speeds when the brakes are applied there is a definite deep bass sound seemingly coming from the rear end. There also may be a very subtle vibration of the pedal. I hadn't noticed these before but then again I hadn't driven it much at highway speeds in the two months or so that I have owned it. The brakes seem to work fine and I just pulled the wheels off again to check and all appears to be well. I also checked the front pads/rotors and all looks good. By the way, the car drove/rode superbly on a recent 1500-mile trip (that's when I noticed the noise). Any ideas or thoughts about what this could be? I had thought of wheel bearings, but the car only has 78K which should be much too early for them to go bad (plus why would they only make noise when braking?). Thanks in advance.