Ray from Rothesay, N.B. has a rumbling sound in his hot water heating pipes. Servicemen drained them twice, thinking it was air in the pipes, but no improvement.
First idea, maybe it is a bad bearing in the circulation pump. You can check that out by putting your ear right to the pump when it kicks on to see if the sound is originating from there.
One of the servicemen said the sound seemed to be coming from the corners, which might indicate some scale stuck in those corners and banging around. If Ray wanted to flush out the system in the Spring that might do some good as well.