We had a huge backup from our basement drain about a year ago (little children and too much paper in the toilet caused it I think). We cleaned up the concrete floor with cleaner and then with bleach and took out the carpet that got wet. All was fine for a while, then the smell started. We get a sewer stench off and on that seems to start in that room where the drain is (it is also the laundry room) then it will spread throughout the house. I also notice it sometimes coming from the 1st floor bathroom sink and shower drain. We also now have these little funny looking flies that hang out in the basement. The biggest number is right by that drain. I have tried drain cleaners and bleach. Every time I pour stuff down the drain, some of the fly things come up. An exerminator that was at the house for another reason saw one of the flies on the wall and told me it was a sewer fly and that I probably had a problem with one of my drains. Can I clean out that basement drain and pipe? Do I have a big, expensive problem that needs a plummer?