Goodness! Could the AC unit have come back to "life" so to speak? Can you turn off the breaker to it to see if that is the problem?

Did that, still making noise. Could it be an underground pipe?

What was it?

Say what? lol

I just moved into a home in February. I heard a loud grinding noise that lasts about 2 seconds, and has happened a few times over the past month. I cannot figure out where it's coming from. At first I thought it was neighbors doing something in garage, but it's not them. It's not coming from the basement or trap door with heat exchanger for basement because I went down there and listened. It's at the upper level, and did it again this morning. It always happens about at 11 am. but it's only done it 3 times over the past month. The A/C is not on, and I don't even know it it works. The heat is not on either. There is a sewer manhole cover on front lawn, and wondering if there is some kind of sewer motor that keeps it moving out? The home isn't that old, built in 2007. I always go look outside and see no one or nothing going on. It sounds like a jackhammer. Anyone know ? (I'm not insane; mother had me tested. :- )

I wouldn't let it bother me unless a voice says you better not be here when John get's here

We have woodpeckers or some other bird that perch on the metal chimney caps in our neighborhood from time to time. It’s really noisy when they drum rapidly on them, like a jack hammer if you’re in the room where the fireplace is. Plenty loud outside, too. A mating sound?!

Sounds like an attic power fan exhaust in which the motor bearings have failed.