Quote Originally Posted by jyardgirl View Post
it could be coming from your heater coil. mine was leaking and when i pulled it there was alot of white dust around it from where the antifreeze had leaked out and dried.
I think Barb's right here, and you probably have a very small heater core leak, not enough to drip but getting things wet inside the duct. When it dries the chemicals in the anti-freeze and maybe mineral in the water are left behind, and dry to a dusty deposit.