Recently in the basement of our home I noticed a nasty smell coming from the floor train. To me it smells like sewer. I heard it is Methane gas in the pipes. It is so bad I had to seal it off until I find out what the smells is. When I sealed it off though I noticed the clear wrap that is sealing the drain off would move when ever the washer would empty, the shower was on or the toilet was flushed. I went to removed the clear wrap and all of a sudden these little nasty black flies were flying out and all over my basement.

What is the cause of the smell? How do I do to loose the smell and now even then flies? I hope there is no pipes broken under the house. All tips and comments are helpful.

Thank you
JB