Sink P traps are a source of some odors as they are full of ugly black, gooey debris, caught up in hair, toothpaste, etc. I take my trap, directly under the sink, apart occasionally to clean it. It is then reinstalled and a double check is done to make sure it is not going to leak. A large adjustable pliers is needed to take apart the fittings and then re tighten them. The P trap is a necessary part of the plumbing as it prevents sour gasses from coming up through the sink, the water in side, acts as a barrier.
The first answer's suggestion was exactally what I would have said. It is just the gunk in your p-trap!
