There's many possible causes for your affliction, but there's only a few you could do anything about. One of them is a chronic infection of the upper sinuses by anaerobic bacteria, which can cause phantom odors and mess with your olfactories. I've had the same problem, although the smell was different. It won't necessarily cause congestion and show up on a scan.
I developed the Flip-Turn Sinus Flush, which will wash out all infections of any sort from your sinuses. The only reason I took a chance when i first did it was that I'd totally lost my sense of smell and felt I'd nothing to lose, but it turned out great.
http://www.medhelp.org/user_journals/show/2322
Keep in mind the procedure is risky, because you have to bend over in a shower, but you can also do it outside on soft ground. Don't worry about the burning sensation from the saline, it won't mess up your olfactories. If in fact it is an infection, you'll get some results in days, but it'll take a few weeks after you stop flushing before the sense comes all the way back.
Go see an ENT about any other reasons for loss of smell.