Six year old 50-gallon gas hot water heater. Heard a slight bang through the walls and thought it was something dropped upstairs. Then heard a steady knocking from the hot water heater. The water meter ticked in unisyn with the knocking. Knocking stops when water not being heated. Hot water immediately got rusty looking. Drained twice... still rusty looking. Cold runs clear. Do you know what it is and can it be fixed? Never flushed it before because I don't know about such things. Appreciate any help.

If it knocks, let it in.

The growling is due to sediment that forms in the tank. The only way to prevent is drain a bit of water out MONTHLY until it runs clear.

The tank could be failing. How old?

In which case, it could be time to replace since you didn't drain it regularly

Water meters also count air as well as water. Not sure about the water heater thing, but the knocking is a big burst of air at the bottom of the tank that has a lot of sediment. When you heat water, air gets released.

DO you have an expansion tank or PRV valve?

Is this a gas water heater? Let me know, Tom

KISS nailed it! It's mineral build up.Assuming that you have a gas hot water heater, If you do not flush,(not drain) your heater on a regular basis it will build up mineral on the bottom of the tank. When the flame comes on it turns the water trapped under the mineral to steam and the steam escaping is what you hear. Let me show you how to flush your heater. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) and some white grains,(lime or calcium carbonate). The whole thing shouldn't take more then a few minutes. Don't forget to flush it out every month. Your heater will thank you for it. You might want to consider cleaning the screens on your aerators if you have a high mineral buildup. If the problem still persists after you have flushed, you will have to drain the heater, remove the boiler drain and go up in the bottom of the tank with a wood or plastic rod, (not metal) and try to break up the mineral and flush it out. Good luck and thank you for rating my answer, Tom

Flushing did work on 4th try. Difference was that I left the release valve open during the draining process. Would this have made the difference or did I just get lucky?
Question... I have read to flush once a month, every 6 months, and once a year. How often should it be done, and is it truly necessary to drain it completely (that's a lot of water)? Can I just drain a bucket a month?

Drain a little until it's clear. No need to drain the entire thing.

Tom says monthly. I do it yearly. Determine your own rate based on what shows up. You may need to increase as the unit gets older.