[Deschodt]

Lately we get woken up around the same time by a strange vibration in our ceiling around 7AM... There's a noise sounding like light jackhammering, like a metal pipe vibrating. Odd thing, it lasts maybe 20 minutes, comes and goes, dies off and picks up again, and is around the same time every day. Early AM.

Data points: the house is on a hillside, so when I say ceiling I mean between downstairs and upstairs in the master bedtroom ceiling, that located is under the garage and near a foundation wall where pipes presumably run. There is nothing going on in the garage above, the compressor is off, the mini fridge runs all day and is not vibrating when I check. I've run there a few times - nada. Can't even hear it. My first thought was "water hammer" BUT a) I turned off water main, drained all pipes, refilled in reverse order etc.. still there. b) no water being used at the time in the house or outside. It sounds mechanical in nature, like a failing motor or vibrating metal thing, clanking, more than water in a pipe but not ruling a copper pipe touching something and vibrating... I really can't think of anything there under the floor of the garage.

EDIT: ALso the heater is not running at that time... Water heater might be but there are no hot water pipes in that area as the bathrooms and kitchen are in the other directions.

Maybe that's when our neighbor showers ;-) (detached home, neighbor is 12 ft away on that side). Maybe it's resonance from something far away... My next thought is to be ready one morning and go turn off water + power when this happens - too sleepy today.

Any ideas or exorcisms appreciated.