I have a Rheem Fury 41V50 hot water heater and an hour ago it began humming loudly. When I turn on the hot water, the humming stops for a minute and then starts again. Is this a sign the unit is going out?

Could it be possible that you have a leaking faucet or valve somewhere in the house? Is the noise coming from the WATER HEATER or the plumbing? Low pitch or high? Would it be possible to find the pitch on a piano? You did say that it stops when you run water, and does not start immediately when closing the faucet. Does the noise happen only when it is burning (heating) or at any time? Is the humming accompanied by pops and hisses? Do you have hard water? How old is the heater? Do you maintain your heater regularly? Have there been any changes in the house plumbing system lately? Are you familiar with your water heater enough to KNOW that this has never occurred before?

Plenty of things that we can't see over the internet. Answer these and give any other pertainant information and maybe we can be of farther help.