We live in a 2 story home built in 2004. Last year we had the basement finished. Since that time we have noticed a strange smell in the stairwell going down to the basement. I originally thought it might be "new" carpet or "new" wood. It smells a lot like natural gas but we've had the gas company here twice as well as a private company and both said it was not gas. They checked everything very thoroughly. We recently did a home energy audit and were told that our basement has a "negative pressure" and a recent radon test showed a level 4. We are in the process of testing again. The smell drives me crazy, not to mention gives me a headache. It is very strong as soon as you open the door to the basement. We live in Nebraska. Any thoughts? Thanks soooo much!

I'v seen negative pressure in walls where the Central Air Intake was sucking air from within walls, not environmental air.
An indicator of this is dust accumulating around, switches and recepticles and their wall plates. Get ahold of some bad drywall?
http://www.cnn.com/2009/US/04/03/drywall.family/

This is not a standard radon test level, how many parts per million did it read. Radon isn't causing this smell but you should explore the results of this test so you know if you have a problem with radon. When did you buy this drywall and from where. The USA imported some bad drywall from China after the big hurricane. Many homes now have to be redone, again in LA.