01 Suburban thump

I have a 01 Suburban 1500 that has a thump like metal expanding when hot. It only happens after truck has been driven and warmed up real good. I notice it when I am at a stop. It only happens very sporatically and not a constant thump. Has anyone had this problem. I borrowed my brother-in -laws Silverado and noticed the exact thing happening. The noise seems to be coming from middle of the vehicle underneath.