All kinds of possibilities here.

Termites can get into a house even over a concrete foundation; they build themselves little tunnels across/up the masonry that protects them. Usually these are in out of the way spots where you don't notice them. Once a year, a large nest will launch a swarm of elites, that can fly off, mate, and start a new colony. Sometimes these will leave the nest inside a house. They are harmless, but of course are a certain indicator there is a nest.

You would not necessarily see carpenter ants, or smell the nest, even if a fairly large nest were present.

White particles in the air do not ring a bell with me as an indicator of any kind of insect infestation. Do you have popcorn ceilings? Generally, what shape are the ceiling texture[s] in? Check the AC filters; if you're seeing particles in the air, they ought to be collecting on filters.

Maybe there was a lot of dust/crap in the upstairs AC ducts. Pull off a couple of registers and have a look with a flashlight. May need to have them cleaned.

Thank-you for your input. We do have termites in the ground outside the house as we do live in the woods but I have (touch wood :) not seen any winged ones in or around the house. The house is a log house w/log ceilings...no popcorn there I don't think...and house is "fairly" new....about 6 yrs old, so in fairly good condition *I think*. My husband thinks something is between the logs of the ceiling and the roof...I think there is a thin layer of insulation above the logs and this is possibly being chewed? We'll check the a/c ducts...at least if we have these cleaned they can be eliminated from the possibilities!

It could be something chewed up and put into the ducts, like mice or carpenter ants building nests. I had carp ants inside a double wall steel garage door, eating the foam insulation. Left white bits at the base of the door.

But why would these particles come from the ceiling? Are the AC outlets on the ceiling? If not it sounds like paint or the aforementioned popcorn ceiling.

Weird.

Thanks for your input! No popcorn or paint as it's a log house. The ceiling is made entirely from logs. I believe there is a layer of insulation on top of the log ceiling and then there's the roof so my husband thinks that something is living in the insulation between the log ceiling and the roof. We did have mice but my husband said he didn't see any debris when he checked the a/c outlets. Yes, the outlets are on the ceiling. The link below shows the type of ceiling that we have...I'm not at home right now so I can't post a pic of the actual ceiling but these give you a general idea....it is difficult for me to imagine where the white stuff is coming from if not from the a/c.

Here is a link that might be useful: looks like this ceiling

Tongue and groove pine, is what you call that. Since it has tongue and groove joints it should not have cracks between to allow anything through. It could be expanding and shrinking and developing some gaps, OR the stuff is coming out of the air vents.