I have a multi-level home with a seperate heating/cooling unit for each level. Over the past month I've noticed an odor on the top level that smells like mild sewer gas. The bathroom on that level gets used and I've checked to see if the odor is coming from the traps there (it is not). Again, it has only started in the past month or so (since the heat has been turned on). I've looked in the ducts and the blower motor area and nothing smells or looks odd. Any suggestions on where to go from here? The other two levels (each with their own units) do not have any odor. I moved into the house in May of '08 and all through the summer I never noticed the odor.