We just bought a 31 yr old house (4 months back) and recently finding out continuous clicking/hammering type of sound when we run hot water from two of our bathroom sinks. It seems like it comes from the wall as if someone is hammering it from inside...the rhythmic tukh tukh tukh.

Recently I felt it, when the toilet was flushed, and occasionally when the shower was on, as well.

Is this because of pipe expanding when the hot waters runs through it?

What should we do to fix? Your help is much appreciated. Thanks in advance.