How can I find the source of a smell?
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.