Last August, after experiencing "hammering pipes" a plumber installed a new water pressure valve since the psi was over 100. When he left the water pressure was 90 psi. However, during the last week we noticed a "groaning" sound whenever we flushed a toilet or turned on a faucet. We called the plumber back and he adjusted the valve to 65 psi and indicated the sound was probably due to "solder in the pipes." The "groaning" seems to be getting worse. If we have not used any water the sound is loud and long when a toilet is flushed or faucet is used. If we use water frequently, the sound is not heard or not as as loud.

What is going on?