Squeaky rear! What could it be?

My 1989 325i has developed a squeak from the rear. Whenever the car is in motion, even very low speeds, I hear a constant squeak from the rear. It doesn't sound like a metallic squeak but more of a "rubbery" squeak, if that makes any sense. Applying the brakes makes no difference. It's rather embarrassing. My Bimmer sounds like a clown car!

I jacked up the car and removed the wheels, then looked at the brakes. The pads appear okay but one rotor shows evidence that the pad is only grabbing near the center. I didn't see any other obvious problems.

By the way, the squeak is not a constant scraping or bearing sound, but the sound of "eek eek eek" over and over whenever a wheel is turning. Same sound in reverse.

Any clues as to what I should be looking for?