I purchased an 18 year old condo three months ago. Since I bought it from a friend I did not have it inspected (and I regret it now). Anyway, since moving in I've had a strange musty smell that I cannot find the source of. I have had the ducts cleaned and have steam cleaned the carpets myself twice. All to no avail. I had someone look at the crawlspace and they said it looks fine.
The carpets look fine on the surface but when I pulled them up on the corners I saw dark discoloration around the perimeter near the walls. Also the corners of some of my walls have a slight darkening to them in the cracks. Maybe just a bad painting job, but maybe moisture behind the walls? There are two small spots downstairs and one upstairs where I can see that water has gotten into before due to an old watermark on the paint (but it is not wet now).
The whole condo smells this way but it is much worse downstairs. I bought a washer/dryer from a family member who had it for years with no problem. About a week after moving it into my condo the washer started smelling musty. I am thinking that there is some ambient mold/mildew or something that is sort of being pulled around the whole place and getting into everything. My couch now smells pretty bad too. The place doesn't seem to be overly humid so that's why the mold aspect is so perplexing to me.
Also the downstairs bathroom has a strange smell to it that I can't describe. Not sewery at all, more like a slight chemical reaction smell.
Lastly, the place was closed up and unoccupied for eight months prior to my moving in. The furnace was kept on but only set at 50 degrees. Also, my condo is connected on both sides to other people and each of those owners say that they have no problem like that in their place.
Unfortunately the smell is not consistent. Some days it's much worse than others. Some days it's barely detectable. I can't find any pattern to it from day to day though. Whenever I have someone come out to help me track down the smell it is naturally in remission that day. Very frustrating for me.
Oh, a side note, I notice that the smell gets worse when the sun goes down. Not sure if the light or heat of the sun is somehow involved.

Help!!!