Odor during defrost on Trane

[second opinion]

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?

[Bentonville]

The time and place of smelling the sour odor is all through the house coming from the vents as the unit is in the defrost mode. The thermostat indicates stage 1 and stage 2 simultaneously and steam is rising from the vents on the unit around the coils.
I have to say that because of the good input I received on this forum, I contacted my installer who was quick to respond. I am very satisfied with my installer because of his desire to satisfy me as a customer.
He first cleaned the coil with a solution and then sprayed it out with water. Then he applied a solultion that began witn an "E". Then he sprayed a mold killer. Following these treatments of the coil, he had me turn on the fan only and allow it to run for thirty minutes before using the heat pump.
My installer said he rarely sees this problem and usually it is not in this time of the year when it does occur. He said that if this doesn't fix the problem, Trane does have an epoxy-coated coil for replacement but I would have to pay the labor. They would pay for the coil.
If there is more input, please contribute. I am amazed that in all of my research about which system to purchase and all of my reading about AC/Heating systems, I never read about "dirty socks". Well, I have had dirty socks and I hope they are gone. KUDOS to my installer for driving the 30 miles one-way and spending about an hour and a half spraying, waiting, talking, spraying, etc. and no charge.
If there is something I should know about all of this, please share more.
Merry Christmas to all here.

[second opinion]

Install a UV light in the apex of the coil

[tinknocker service tech]

Install a UV light in the apex of the coil

agreed uv light for sure