I have large amounts of sewer gas escaping from basement floor drain. The trap has water in it. I have flushed it out for 5 minutes with a hose ( while this is happening a large amount of gas escapes) after 2 days the gas is escaping again(from the trap) and the water in the trap looks nasty again. The gas odor is strong and undoubtably coming from the drain. I have repeated this 4 times in the last 2 weeks. 3 days ago the local sewer district came out ( after I already poured water down the drain and the gas was not as strong) and said because it is not overflowing or no other pluming in the house is backed up that the problem must be in the house and not with the city sewers. I wanted to see if anyone had an Idea on what this could be before I called a plumber so I would know what I am getting into? Does this sound familiar? Any Ideas?