1. I don't know exactly how to state this...I have a gas water heater approx. 5 years old.....in the garage of a two story house. I have owned the house since 03', recently when the water in the house anywhere is turned on, "sawing" sounds are coming from the pipe attached to the top of the water heater that leads into the house. Sounds like the "shooting of water through pipe", on, off, on, off, on, off...this runs until the water is turned off. The noise is at the top of the tank, not heard anywhere else in the house.
I have flushed the hot water tank with all faucets open.