Also no natural gas lines
Well then, that rules out the most obvious answer.
Another idea...check your A/C plenum (I think aka as a distribution box) in your attic (or where ever it's located) for moisture build up. If it is holding moisture you may have a mold problem in your A/C vents.
Another idea...if you have a drain that you rarely use the p-trap may be dry and sewer gas is backing up through the drain into your house. This is an easy fix. Just use the drain.