Once or twice a day I hear a low frequency pulsating hum. Sounds mechanical. Sometimes it does not pulsate. It can last briefly or for as long as 5-10 minutes. Whether the water is on or not. It's inly been happening the last couple of weeks. Mostly during the day. We narrowed the source to an area between two bathrooms and under our second has furnace upstairs. And only heard here. Help

Please explain again, "an area between two bathrooms and under our second has furnace upstairs."

I would guess the furnace. How about the fridge? Mine hums in two tones, and I hear it best in the bathroom next to the kitchen, more so than when I'm in the kitchen.

We actually figured it out. When my neighbor uses her washing machine on the filling tub and when the water is drained we hear the distinct noises. It's only been the last 2-3 weeks. Her house rattles and mine is picking up noise and vibrations through city pipes.

City and plumber are coming out to see where in her line the problem is.

Is this, by any chance, a duplex, townhome, or condo? If it is two separate structures, I can't imagine the sound traveling between them in the city pipes.

It is in fact two separate houses. But only separated by 50 ft. City was out today and said that he couldn't detect any issues outside but believes their pressure regulator valve is broken.

I know, he agreed he didn't think we should either. But I can tell by the length of sound and whether it pulsates if she's flushing, running dishwasher or washing machine. Never happens in shower or faucets. She was gone all day, I heard the noise for about 5-8 sec and texted her if she was home. She said yes just flushed and asked "Did you have noise" to which I replied yes.

WOW! This has to be one for the books. Please keep us up to speed as to how this plays out.