At times, there's a rattling or banging coming from our water heater.It appears to come from the cold water supply line that enters the water heater from above. The copper supply line from the wall connects to a copper or brass gate valve, then to a flexible copper pipe and then to the water heater inlet. I can stop the rattling/banging by holding a metal object against the flexible copper pipe, but of course I wouldn't want to do it 24/7. Any ideas what may be causing this?