I hear noise in the back of my truck were the.

Have it inspected at a tire and alignment shop. I can't hear the noise from here so I can't tell if there's a loose bowling ball in back, a shock absorber mount is broken, the spare tire is loose, the tail pipe is rattling against the frame, a wheel is loose, or if someone left a window open. You are in a better position to identify the cause because you actually know what it sounds like and where it's coming from.