I have a draft occurring in my 2 story home, usually on windy days. The main floor is fine, but in the master bedroom on the second floor, there is cold air entering the room from the air return vent. It's not much, but noticeable. In the basement (which is finished), cold air is coming from the heat registers, quite noticeably, and when the furnace isn't running.
I've made sure that all the obvious sources for drafts are sealed (doors, windows, etc). The house is ~15 years old and the heating system is forced air.
Can anyone suggest where this air might be coming from?