lately when i reverse the rear sound like a air horn for a few second it dosent happen all the time but every other reverse it does it, could this be the caliper sticking or the break pads worn down? before i go spend money on the wrong parts i was wondering if anyone experiance this sound and fix it

 

I just started getting this noise. I'll wait until installation of the rotors since I have a pulsation during breaking and small shimmy. Then again, I did recently resurface the rotors and install new pads. Not sure what your condition is...