My 1993 500E with 115,000 miles has a had a periodic and very subtle humming sound from the front, that sounded too high in pitch to be from power steering, since I bought it at 110,000 miles. It would appear for a matter of seconds disappear and then reappear.

Today, I noticed the sound to be much louder and almost constant. I then noticed that, if I lightly tapped the brakes, it would abruptly stop. The front brakes have about 25-30% material left, and the rotors will be due for replacement when the pads get replaced. There is no pulsing in the braking, which is altogether smooth. Any thoughts appreciated.

P.S. To those of you who helped me with my previous battery drain problem, I've not gotten back to trying resolve it and been using using a very nice little shut off switch on the negative terminal as a temporary resolution.