what does it smell LIke? something could have died under there in which case you would need to hire someone to get it.
However, I had a problem similar where every time I passed my kitchen sink it would smell like bad breath. it was the most bizzar thing. Also, when I went into our basement, it smelled worse so i knew it had something to do with the pipes.
I don't know if you have a basement or not, but my neighbor explained to me that since we have a full bathroom in the basement but never use the tub, water sits in the Ubend and festers. and since all the pipes on that side are connected, I would get a whiff of it upstairs sometimes. So if you have any sinks or tubs that you don't use very much, fil them up and drain them and run the water in them every so often.
Hope you find it!