While I am here, I might as well ask another silly question that has been bothering me. From time to time when I flush the toilet or run the water in the sink of the bathroom, I will hear a gurgle sound in the tub and then there is an odor of sulfur or rotten eggs coming from the drain. I have never experienced this in any other place except for this one... and it is a 2 family where I am in the second floor unit. What causes this and how can it be remedied?