clunk when backing up

when i back out of my driveway i have a slight clunking noise under the truck when i hit the brakes. then when i pull to the end of the road at the stop sign it does it again, then not again until i park and back up again.

is this the rear brakes adjusting themselves? or something i should worry about?

 

it's your front brake pads. when you change direction (forwards and reverse) the pads move in the calipers. when you have been driving forwards the pads are forward in the calipers. when you put it in reverse and are backing up as soon as you touch the brake the pads grab the rotor they are thrown backwards and hit the other side of the caliper. the opposite happens the first time you hit the brakes back in drive.

 

could be wheel bearings but more likely its just the floating brake pads