Pipe knocking

Hope someone can help me troubleshoot this:

I have a 1 floor house in texas, built circa 2005. Recently I've been having issues with a rattling pipe which I've been unable to identify. Here are the issues:

1. When someone turns on the water in the master bedroom, a pipe somewhere in the attic rattles. It starts off with a knock every 1-2 second or so, slowly speeding up and dying out. It usually rattles for about 20 seconds or so. Then, it's quiet.

2. It seems to be associated with the hot water, the best I can tell, and the master bathroom. The master bathroom is fairly far from the water heater, so usually wife leaves the water running for a while to warm up. So I am thinking it's thermal in nature. The water heater is gas, attic-mounted as well.

3. I tried to catch it in progress, but whenever I sit around in the attic waiting for the rattle, it just doesn't happen. If I try to make a run for it when I hear it, by the time I pull down the ladder and the ladder's spring quiets down, it's gone.

I realize there is not much to go by, but what would pros think of these symptoms? The attic pipes are easy to access, and I tried moving them by hand, and don't seem unnecessarily loose. I have a general area identified, and the few places I've checked under the blown insulation, the pipes go through 2x6s, and wrapped in some plastic clamps inside the beams. There is a few mm play in some cases.