My guess is that it's something in getting the fan spinning. One way to test would be to jam something into the fan blades (with the power off) so they don't turn at all to see if it changes the behavior.

Ok...I'll admit that sounds weird, and I can't suggest what it is, but I don't think it is major and I doubt it is something that would cause a fire. I also can't imagine it would be static related. If it were just the fan, I'd say the motor is starting to fail, but I can't account for other things triggering the noise. It's good you have an electrician coming. Please don't stress out so much. And re-post when it gets resolved. I've never heard of something like this...

Perhaps bearing or bushing needing lubrication? Let somebody take it apart and see if they can easily turn the fan & motor by-hand. Some well-placed oil might solve it (at least for now).

Do you by any chance have a sub woofer?

No, no subwoofer.

Perhaps I was unclear. It's obvious that the fan is not the problem. The sound doesn't come from the fan, it seems to come from the attic or somewhere in the walls when different switches are flipped on. It just so happens that one of the things on those switches is a fan and I noticed it there first. It also makes the noise on a hall light and when the thermostat clicks on, all of which are close to each other and likely on the same circuit.

I can't recreate the sound at will, it doesn't happen every time or even every day. So hopefully the electrician can still figure out what is going on.

I read elsewhere on the web about "arching" being one possibility. Or a ground wire accidentally touching a hot in the panel box. Hopefully it's not too expensive of a fix and isn't dangerous. Thankfully we have Nest Protect smoke alarms that will notify our phones if there is smoke when we aren't home, so that makes me feel better.

I mentioned static because I live in Alabama and this just started happening at the same time as our coldest weather yet. Everything is super dry and cold, and my heat has flipped to auxiliary a few times. Not sure how/if it connects, probably not, but that was my thinking.

The electrician should be here tomorrow afternoon and ill update after I know what the issue was. In the meantime, any more suggestions would be appreciated! Thanks!

Jen

Well, AFCI were specifically designed to detect arcing issues though if there are multiple things that cause it and it only happens when first turned on, I'm not sure that an arc is likely.

If all these things are fans or ceiling lights in the top floor, it's also possible that you've got some kind of thermal issue going on in the attic.

Thanks @ronnatalie

The fan and hall light are upstairs. We have a switch for the hall light down stairs as well (it lights the stair well). The thermostat is downstairs but located on the wall that supports the stairs.

I'm thinking it's something in the attic. That seems to be where the noise is coming from. I'm going to try and get a video of the noise in case it won't do it when the electrician comes. Hopefully we can sort out what is happening.

Just leaving an update before I forget.

All of the lights/fans mentioned are on the same circuit, along with a very old electric fire alarm. The fire alarm was original to the house, built in the late 1970s. When something on the circuit was flipped on quickly, a surge of power went to the fire alarm and caused the weird low pitched chirping I was describing. It seemed like it was in the walls or attic because that is right where the fire alarm sits. I just never in a million years would have thought of that pesky thing.

We had the electrician take out the fire alarm and seal the wires. We patched the wall so it is totally gone. We have "Nest protect" wireless alarms in our home that we are very happy with and have never relied on this alarm. The electrician said it's funny how these old electric alarms are actually quite a fire hazard themselves.

The charge was $75 but it really worked out for us because we have been talking about getting rid of that old unsightly fire alarm for a while now.

Hopefully this helps someone out there!

...take out the fire alarm and seal the wires. We patched the wall so it is totally gone.

I hope that when you say "seal the wires" that you mean that they are decommissioned from the source and are no longer connected to the electrical system. Because if you patched the drywall over wires that are energized with just wire nuts on the end, then that's a code violation. All wires must be terminated in an accessible box.

GWJen-
Thanks for the update...fascinating.

Most likely the turning on of other devices dropped the voltage enough to freak out the fire alarm, rather than causing a surge of some kind. The fan would be the most likely to do that, due to the current required to get it started spinning. Glad you found the problem.