The hot water heater is:

Bradford White Model No: RG240T6N Serial No: SK42274482 Says: "Defender" "Equiped with Hydrojet" Installed: 11-27-2018 Natural Gas 40 US Gal 40000 BTU

House is 100 years old. Kitchen and shower are cartridges. Everything else is screw type faucets. Almost all galvanized pipe. No water softener or filter, other than right before fridge supply, so we do have a problem with mineral deposits. I have to change the rubber washers and scrape the seats on the bathroom screw type faucets yearly. Same with flushing anything with an aerator.

I am a general contractor, so I have skills, but plumbing is not one of my personal strong suites, more because of lack of knowledge, than anything. I'm comfortable doing most things with guidance, though.

There is a whistle coming from the inside of the hot water tank about 6" from the top, about the same height as the relief valve, directly above the gas valve. I can hear it throughout the house plumbing, which makes me think a clogged valve or connection inside the water heater. I only hear it when a faucet is open, it goes away when the faucet is closed, and it gets higher in pitch the more the faucet is opened, but gets almost silent when the faucet is open all the way. It does not sound when heat is called for in the tank, or at any time while it is heating. There is hammering sound in the pipes in the basement where the heater is, when a faucet is closed. Pipe movement is very minimal, if at all. I heard and felt hammering in the 2nd floor (highest) bathtub hot faucet for a day or two about a week ago. It did not go away after running the hot water on full to fill the tub that night. After a day or two, it wasn't there at all, never getting less and less, just gone.