A this is going to drive me crazy question. Currently I pick up side jobs. I am well experienced. Got a call about a 80 gallon water heater in a 3 story Townhouse making a vibration noise in a wall. Sure enough, in a wall where they ran the pipes to upstairs there is a constant vibration noise almost like a motor running?

I traced it back to the water heater (about 15 feet) which is 5-6 years old. No motor is hooked to the lines. No air in the line, opened the 3rd fl faucets (plus the others). Water heater supply like (3/4") has a level handled ball valve. Street water pressure is (I think was) 55 ft lbs? There was another thing I found and that was the hot water moving up the cold water pipe. I drained and flushed the water heater. Checked the temp. (2) control (120) and put a expansion tank on the inline. Also replaced the pressure relief valve. All was fine for about three months. Got a call today, it's back? I can only think of putting a check valve on the line or a pressure reducer? Any other suggestions? I also want to see if I can reach the pipes (Cubby hole) and see if they are loose and can be shifted? Any help appreciated.

there is a constant vibration noise almost like a motor running?
Are you saying that the pipe vibrates even when there's no water being drawn? Do you have heat trap nipples ,(see image) on the heater?:Let me know, Tom

No water running should mean no noise. Even a slight leakage at a valve or flush valve can cause a vibration but that would be cold line. Circulation pump or yard irregation system present?

I didn't do the install so I don't know what nipples are on there? It's hard piped 3/4 copper (no flex piping).

It's strictly the hot water side. Cold water side has no affect. Run the water it stops until you shut the water off and it starts again. There is no Circulation pump. It's Florida so the A/C is also a heater. The only pump in the location is one meant to use for draining the water heater? The drain line (also copper) is closed off? Never saw this before so I didn't play with it. When I drained it I hooked a hose on. The vibration sounds and feels like a electric motor? There's no access door to that area so I doubt there's a motor sitting in there?

Greetings from Sarasota, What part of the state are you from?
A pump on the heater drain? That's a new one.
You say the vibration starts when the hot water's not running and stops when no hot water's being used? Interesting! What happens when you shut off the cold water supply to the water heater and open a faucet to bleed off the pressure? Does the noise stop then? Let me know, Tom

The water heater is in Boca Raton. Yes they put a Y adapter on the drain so one side has the reg. drain spicket, the other goes to the pump. The out of the pump is shut off by a valve but the power is on? Motors not running? Would it run if I open the valve? That's why I left it alone.

Trying to recall from a few months ago.. I think I opened both side of the faucet to bleed it? 3rd floor had air in the line but not after.

When you use any hot water the noise subsides. Shut off the water and in just a few seconds starts again (builds in sound). Personally I would just leave it. The sound comes from the wall next to the entrance. But she says she hears it throughout the home?

It's nothing like a knocking like air in the pipes. It's more of a constant humming that can be felt when you put your hand to the wall. (like a motor)

Let me try again. What happens when you shut off the cold water supply to the water heater and open a faucet to bleed off the pressure? Does the noise stop then? Let me know, Regards, Tom

Sorry for the late response. I am trying to get there to check a few things out? She had said she shut the circuit breaker off (?) and the noise stopped as the water cooled. She turned the CB on (next day) and so far the noise hasn't continued? First thing I will recheck is the thermostats temp. 120 for both? Maybe knock it down to a little lower?