OK we moved recently, it's been less than six months. I don't know if it matters but the weather has been its warmest sense we've been here. I think it's relevant because this smell just started about two weeks ago. The smell is only in my room, it smells like it's coming from the two vents, one under the window and one in the bathroom. The bathroom does have a spot on the floor with some pretty tough mold about three or four feet from the vent. When the heat was on you couldn't smell it, but now that the air is on you can, which I thought was weird because it smells just like spoiled milk, or maybe some type of weird mould, but definitely not a dead animal. I did just have some people move my new bed frame in by the window, about two weeks before the smell started. I looked in the vents, down just before they turn, nothing there. I smell it the most when I'm setting on the edge of my bed, the edge by the vent, and at my desk, which is by the the bathroom door, which is where the vent air flow is. You can see why I think it's the vents. But I'm confused and haven't called any professional because I don't know if that's really where it's coming from, because of the stuff I said before and because I haven't smelled it anywhere else in the house, but I spend the most time in my room. I guess that's about it. I just want to find the source, I can take it from there.
Had the same issue in my home with a brand new HVAC system installed. It always happens when shifting between heating and the cooling functions. I don't know the "why" but I do know that turning the speed of the fan motor higher solves the issue. No clue about your HVAC but mine has an electronic board that has a bank of tiny switches under the front cover. You can change the speed of the blower motor using different combinations of on/off with these switches. You'll need to find your HVAC manual and the section that pertains to the blower motor speed. The switches and writing are super tiny on these boards so get a magnifying lens and go to town on it.
Make sure you turn off the switch that sends power to the HVAC before you start messing around with the switches!!! Most modern homes will have a light switch of some kind right next to the HVAC system.