I just moved into a townhouse. When I spent a night there a few weeks ago, I didn't notice any odd noises. Now that I am moved in, I notice lots of banging/clicking/fluttering noises that are present when NO water is flowing, the noises are even at night. It sounds like rattling check valve flapping around in the housing (if this was industry) or even a busted vane of the water meter. Sounds almost metal on metal because I can hear it clearly on all three levels, but it's loudest at the meter/regulator. But why is it making noise when the water is NOT moving? The townhouse is on public water and has a pressure regulator. I've checked for leaky toilets and garden hoses, but no water is running.

Any ideas on things that make noise in pipes without flowing water or temperature changes?
Thanks