I had a very similiar problem a few years ago. Tried everything I could think of to no avail. Then had someone from the Board of Water Supply stop by and explained it to him. He had me install a pressure guage (temporary $10.00 one to screw on) on a faucet and check my pressure from time to time. Found the pressure regulator to be defective- it would allow the pressure to gradually build up in the system and the pipes would make a hammering sound, which got worse with time. Replaced the pressure regulator and the problem was cured.