Another pipe noise

Hi all. Being a retired auto mechanic I am able to do my own plumbing and electrical work at home, but this one has me stumped.

For no apparent reason, occasionally I get a moaning vibration hum in my water supply pipes with nothing running. I assume its air but I don't know. It can get rather loud at times.

It might happen once or twice in a week, or once or twice in a day.

I have no idea. Cant see how air would remain in a water supply pipe. I can alter or stop the noise by turning on the bathroom sink on the second floor sometimes.... but, not so that I could say I've found a source of the problem.

Then I thought water seeping, vibrating would be more likely than air in a water supply line.

Maybe a toilet... but I don't see any signs.

A year or so ago I installed a pressure regulator at the water supply in the basement just after the main shut off because water pressure of 115 was playing hell with a kitchen sink "o" ring. I regulated that down to 75.

I did get a little noise at times from the regulator, so I stuck a small water hammer arrestor in line, parallel on a tee fitting. But that was a year ago.

Any ideas? Thx.

after the moan noise, when you turn on a faucet does the first bit of water come out like it is higher presure than it should be ?
then calm down to normal ?
sounds like your pressure regulator is bleeding thru,,but puzzled where excess water is going

you have home work LOL

go look at your water meter, with everything in house OFF

look at the dial, you should see a small red triangle looking thing

is it moving ? ANY ? if it moves ANY, water is being used

record or mark with sharpie, location of triangle.

get a cold drink and come back look in 30 minutes

do not use any water during the test
has it moved ?

Thanks for your reply. Just cked, the meter didnt move at all.

I had a wild thought, if viable it may provide the cause, but not necessarily the cure.

My 15 yr old daughter likes prolonged hot showers. Must be 50 + feet of pipe to the shower on the second floor, don't know... maybe a lot more. Anyway, the hot expanded water in the pipe cools after the shower valves are shut for a while. The cooler water takes up less room in the pipe, pressure may drop some, and maybe provides a place for a little extra regulated water to seep in, making the vibration in the pipe from the regulator???