Not much to go on. What kind of noise. pipe vibration, whine? Could be air in the line. If you are running the pump, it could be pipe vibration. If it is from the valve on the faucet, you might needto check yourvalve stems, etc. Does it happen with any hot water valve?
I have a "singing" noise from my water heater when ever a hot water tap is flowing. I was told it was a bad check valve on the water heater outlet. Or, maybe it was the inlet? In any case, I will be chasing it down this weekend....
Had the same problem a couple of years ago. The check valve on the hot water heater outlet whistled whenever the hot water was flowing. Dealer replaced with a new one. It whistled too. Dealer replaced with a shutoff valve--no more whistling. Just make sure you put valve in for the correct direction to stop flow. Dealer put mine in backwards and couldn't bypass for winterizing until I turned valve around. I wrote about this in the old forums.
Dale & Carol
2010 CF30SK Patriot