I read some other posts about pipe noise, and it does not sound like what we are experiencing. The noise sounds like a ship's horn (low, deep, lasts about 15 seconds). It goes off every 5-10 minutes.
I think I've eliminated it being an AC/heater problem. I can't hear it from the attic or outside. Our gas hot water heater does have some condensation in the drip pan, or it might be a leak. The sound goes whether water is running or not. Any ideas?:confused:

First open the lids on your toilets and check the water lever, The water level in a toilet tank should be 1/2" below the top of the white overflow tube.
If it's above that or right at the top you have located your noise. Lower the tank level back to the normal level and enjoy the quiet. Regards, Tom