I have it when I run hot water through my pipes.
It's the copper expanding.
My pipe run over I beams, up walls, near HVAC ducts - the works - so it could be a noise against any of these or just the pipes themselves getting bigger with the heat.
It's a tap tap tap.
I'm willing to bet yours might be the same when the heater fires up, as long as we are not talking a BANG BANG BANG here.
I could think of ways to get rid of it (e.g. flexible lines coming off the heater) but wouldn't really bother if a little bit of expansion is all it is. Anyway, whenever I have tried to fix mine, the taps just start up elsewhere!
Insulating the pipe might reduce the noise as well.