We moved into a 40 year old home a year ago and have had a foul odor throughout the first floor and second floor hallway. We can't identify the source or what the cause is. We think it is in carpeting or possibly walls, but are baffled. Seems like a stale cooking oil smell, although I'm not sure that's the best resemblance It is more severe when the central air is not on. Any suggestions on detection and elimination short of ripping out the carpet (only to find that wasn't it)?