Pipes Popping/knocking

I just purchased a 2 yr old house and have a popping/knocking noise in a wall above the hot water heater whenever water is running. This occurs no matter what water is running to, shower, washer or dishwasher. I do not think this is 'water hammering' as it only occurs when the water is running. It also seems like a water pressure issue as their is a pressure tank (or what looks like a pressure tank) on the hot water heater. It has gotten louder or the last week or so.

Any suggestions on what this can be will be greatly appreciated.

Thanks
Steve

It could just be the pipes expanding as the hot water flows through them. If they go through a wall or joist and are a little tight, it could be the pipe rubbing on the hole when it expands. Pipes expand in length when heated, too, so an end could bump up against something at an elbow, if it is tight against a joist or wall. My unprofessional opinion.