What could cause banging noise in hot water pipe lines whether the water is running or not?

The noise happens when the hot water is turned ON wherever in a two level house.
The banging noise also happens when NO faucet is running.

We have installed an expansion tank above the gas water heater to no avail.
I lowered the hot water temperature on the water heater to no avail.
The banging noise, mostly along the hot water copper pipe line in the basement ceiling, coincided with a replacement of a valve on the cold water line, and change in gas pipes but we cannot tell if there is any link and how.
Anyone has any idea why?
Thanks-
J.

When is the last time you flushed the water heater? How old is the water heater? What type of heating system do you have?

Last flush? I don't know
We have gas water heater and it is 3 year old max.
I have read about deposits in the heater but I doubt this is the issue since the banging noise started almost exactly after the work on cold water valve and putting larger gas pipe. The gas was cut for a day or two for the contractor to do the work and we used the hot water both days. Would you say this is system wide pressure change?