We have been having this issue of the phantom, what I was describing as wet a formaldehyde odor for the entire 3 years we've been in our current house. The odor would come and go with no apparent cause. You would go I to a room and it would hit you square in the face, like someone sprayed a 'mouse urine soaked insilation' scented air freshener in there. We would give it an hour or so and the smell would slowly fade. Sometimes the smell would stay gone for weeks or months. The issue was predominately in just two rooms of the house, but has made single apearance in a few other rooms, but not all. I tried to figure it out but I couldn't find anything the rooms had in common. One room is on the first floor the others on the second. One rooms is on the east side of the house, the other on the west side. One is a bedroom the other is an office. Neither room is near plumbing. I spent an entire year tracking the smell, recording what room it was in, who smelled it first, who had been in the room last, how many days since our cleaning lady had been there, inside temp, outside temp, cloudy or sunny, humidity, wind direction, wind speed, how many days since the last rain... After all that I couldn't isolate any pattern or specific condition. The only thing I thought they had in common was the HVAC,and the problem only seemed to plaque us during cooling season. We spent $2,500 on a new cooling system only to have the smell come back a week later. The worst part about it is the randomness, there's no way to have a Profesional come out exactly when it's happening. We were defeated. The next step was to tear out the carpet and pad under the assumption the previous owners pet urinated in there and the smell was in the subfloor. Thankfully I tride another round of Internet searching and found this post. After reading this post this morning when the smell had reapeared, I started taking apart and sniffing every outlet and switch in the room. Finally I got to the light fixture and *ding*ding*ding*, we have a winner. The back of the bulb socket smells like the source. I then checked the house and every room we have had the problem in has the same fixture. I'm off to get new fixtures and hope the problem is resolved. Fingers crossed.