Trane 16 SEER dual fuel system. The exhaust is a part of the unit as far as I know. The unit is outside and I don't see how exhaust can get in the duct system. I only smell the odor when the thermostat indicate stage 1 and stage 2 simultaneously. Steam rises from all the vents found all around the coil during this operation.
Lets clarify, the time and place that you smell the odor is at the outdoor unit during defrost when moisture is coming off of the outdoor unit, correct?