mp: RaceFanOne has got the right idea. What ever you are putting down that drain is coming back to haunt you. Dead skin cells..crud from mouth washing..sputum..you name it. It all lies in the drain line fermenting and generating a life of it’s own.
If measures are not taken soon, this evil manifestation will continue to develop until you will have no recourse but to move from your present habitation, or have the priest come by to do an exorcism.