Has anyone come across what sounds like humming (Not water hammering) in the water supply lines in a home. Even with the water off, it will sound like Tibetan monks at a morning gathering. And if you have what is the problem and fix for something like this. Thanks,
Don
The anti-backflow valves on exterior faucets can make a humming sound when the water is first turned off for a few seconds.
Is it occuring all the time?
Make sure the water heater TPR is not discharging a little bit somewhere.
Does it have gate valves? If the gate gets loose in the valve the least little bit of flow can send one chattering. Oh sorry, plus what Bruce said.
What Jack said.
I had one a few years ago that only hummed when the water inside was turned on, but it was at all faucets, tubs, showers, etc. Since I have had some experience with those nasty gate valves, I suggested to my Clients (and the sellers, who obviously needed to find another plumber) that it was the gate valve. They used my plumber who replaced the valve, solved the humming problem, and told them that every gate valve in the county should be replaced.
If there is humming, there has to be flow somewhere. If there are no visible discharges, and there are under slab water lines, then it could be an underground leak, which could cause serious structural damage.
Jim King
Good point, water is moving if it is making noise. What kind of heating system? Do they have a return on the HW line or a pump. Hopefully they don’t have a leak. I chased a noise once in the water system, seemed like forever, and it turned out to be the transformer for the door bell attached to the same piece of wood as a cold water line. The hum was picked up by the water pipe and it sounded like a plumbing problem. Drove me crazy.
Are you in the San Fransisco Bay Area??? There have been recent pro-Tibetan protests and it just may be a morning prayer gathering.
Toilet valves can hum when partially open with tiny particles.
It causes the sound to travel upline for some ways.
try singing a few lines LOL
Toilets are almost always the culprit if no water is being used, like John said.
The older type ballcocks with the long float rods.
Easy to check out, just push down gently on the float to locate which toilet is doing it.
I had a service call on one that was pretty loud that really frightened a lady one evening, she thought something was going to blow up.
Thant can be a bear to locate sometimes, any of the faucets or valves could have a torn washer that vibrates as water passes over it, causing the moan. as mentioned earlier toilet ballcocks, main gate valves, are some of the most common culprits. I just did one the other day and it was the cartrigde for the master shower.
So pretty sure i’ve got your answer. Do you have a smart water meter? They send ultrasound through the pipes to check throughput so even if the water is off it’s still going… the little bitty wavelength has a BUNCH of resonance frequencies which is my guess as to what you are hearing. There are parts of my house that I can hear it going crazy but no one else can and I can even feel like a resistance from the vibrations… I have the misfortune of the entire surrounding water table and drain system having the sound course through it at all times, varying depending on what interference and subsequent resonances that has ._.
Good info, Nathan.
And, welcome to our forum!..Enjoy! 
Were smart water meters around twelve years ago
well crapola i didn’t realize this was 12 years old 
yeah i don’t think smart meters were around back then… woops… what was the solution?
Dumb meters…LOL
Those are SPY frequencies and you need your tinfoil helmet to counteract them!
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They do have ultrasonic based water meters but you won’t hear them resonate sound through the pipes unless you are also picking up radio waves with your metal tooth fillings 
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Old thread, Simon. From 2008. I am sure the monks have relocated. 