Hello everyone, I need some help. I have a problem with a mild sour smell in my house from time to time in the bedrooms/living room on the main floor of my bungalow. It‚Äôs slightly annoying, but that‚Äôs all. I have forced air/heat with an electronic air cleaner and an Aprilaire unit. I only notice the smell after switching over to heat and in the early mornings it‚Äôs most noticeable. Sometimes I get the smell when the furnace isn‚Äôt even running. I do not have the odor when I run the central air or in the summertime. The basement and upstairs don‚Äôt have the smell and both are dry. I was wondering if there is something I can blame the problem on? The ducts were cleaned 8-10 years ago. I do not know if the old carpets are the problem, although they don‚Äôt smell at all from what I can tell. I also don‚Äôt understand why I would still smell the odor when the furnace isn‚Äôt even running? I want to track this down and make sure there isn‚Äôt a larger problem. I appreciate any advice that can direct me to a solution.