For a while now, I've had a loud creaking noise from my left rear brake caliper. It happens when the car is standing still, and I push and release on the brakes. It's almost as if the caliper is flexing back and forth. I haven't had a chance to inspect it yet, but could this be due to worn caliper bushings? I just put some solid ones up front yesterday, and plan on doing the rears once it's time to change those pads. Is there anything else it might be?