This has happened a few times within 3 weeks or so. When I'm NOT applying my brakes, I sometimes hear a loud type of humming and actually feel it coming from the rear (disc). If I apply the brakes, it stops humming. I'm thinking about changing out the caliper on the rear driver's side because it gave me some problems when I tried to compress the cylinder in the last time I put brakepads on. I did get it compressed but it was way harder to do than the other 3 calipers, this was with using a large c-clamp too. So do you guys think the piston in that caliper is freezing up occasionally which is causing this?