Our basement has developed and awful smell. I can't figure out what it is. It never used to be like this. It is very clean. No moisture. Cement is bone dry. It has a sump pump. The furnace, water heater and water pressure system are in this room. I do have an exhaust pipe fan that pulls air out of the basement (1 and 1/2 hours a day) to the outside so you would think that this would not happen. Has this happened to anyone else?