 Usually it's bacteria. You get the bacteria that will decay in those drains, and it will cause what's called a biogas. And the best way to clean this is to really take the drain completely apart. That's the trap that's underneath that. And to really do a good job of scrubbing it clean, I would use some oxidized bleach for that as well. If you can get that super clean, that will kill the bacteria. And if you can even plug the pipe and sort of fill it up with some of that bleached material,