I have a sink which emits an odor from the drain. I checked the p-trap and there was a pretty nasty buildup of black slime in it. I'm cleaning that out, but find it strange that just this one drain has this problem. Another right next to it is fine. The drain with the problem is the highest drain in the house. The vent for these drains seems to be fine, but sometimes I do smell the same odor coming from it. Any ideas on the cause of the slimy buildup in the p-trap?