What could be causing my hot water pipe to rattle?
It's not hammer time, it's thermal expansion. As the hot water travels through the pipe, the pipe itself heats up and expands. As this happens, fasteners and any materials in contact with the pipe experience friction and, at times, the pipe moves slightly against that material and causes a poping sound. It's like snapping your fingers. We hear this all the time with other materials in other circumstances if you think about it. The frequency of the poping changes as the pipe heats up. The fix would be to better secure the pipe (not tight, but with an intermediate material) to the studs or block as necessary to keep it from rattling or "poping". If attached to concrete, insulate the pipe with something, anything, to keep it from contacting the concrete.