Being home more today and the outdoor temp being colder, I noticed my Trane dual fuel unit ( installed this summer) has defrosted more. We noticed a strong, almost sour odor when the thermostat showed stage 1 and 2 simultaneously, which is defrost. The air was blowing through the vents during this time and the odor is not so pleasant. It dissipated after a couple of minutes when the defrost cycle finished. I have mentioned this on another post before and was given good input as to the smell being sort of like opening a freezer. However, because of the strong odor, my wife asked for reassurance. Naturally when I contact my installer during the week, the unit is normal and everything is as it should be. it probably is but I would like to ask if this odor is normal. Thanks and I wish all of the great contributors here a Merry Christmas!