Is it possible for you to pull it out and hook it up to the house to test for a day or two? Some models have a fan on top for ventilation that may account for the fly noise if something was in there or it hadn't been started in a while. There are threads on the forum on how to pull out some models, fairly easy.
2006 Ford E-350 V10 Agile 4x4 SMB EB50 - Sold
2000 Ford E-350 7.3L Quigley 4x4 hard top with custom interior by Xtreme Fab - Sold