2004 Chevy Silverado making noise at 15 mph

there is a noise coming from the rear when you drive slow and once you get to 20 mph the noise goes away. i have replaced the rotors and brake pads and i also removed the parking brake linings to see if that was the problem but didnt make any diffence and then reinstalled them, also put a new caliper on the right side. when putting new rotors and pads the caliper didnt extend out but you could push it in.i jacked the rear up and then ran it in gear and then it didn't make any noise til i hit the brakes, but when the truck is on the ground it makes the noise.
so i was wondering do i have a wheel bearing problem? or brake problem?