As a Property Manager for Commercial Office buildings, one of the Office Suites has dead gnats (black, very small with wings) coming out of the AC duct-work and dropping on desks. There is no food or garbage in the Suite and no floor drains or water source. Any ideas where the gnats might be coming from? I have not seen water leaks on the drop ceiling tiles or around the AC units that are also above the drop ceiling tiles. Pest Control Company has not found the source of the problem. Any suggestions?