Unknown noise

Wife’s 2000 Malibu has a noise like a brake pad hitting a bent rotor. Or like an old rear drive universal joint going out.

Many mechanics have looked and have replaced brakes, bearings, rotors, and axles. Nothing has made any change in the sound. Ideas?

Does she have to hit the brakes for the noise to be heard?

No, using the brakes makes no difference in the sound.

I might add, we check for out of round wheels and any rub marks between the outside back to the motor. Nothing - everything looks good.

Do calipers get bent?