My name is Mark. I have a Rudd Furance forced air. Was good for many months. Now every time AC goes on ,I have water on side of drain pipe. Can not find out where it is coming from. My pipe has a plastic elebow on it

Sounds like you have a restricted condensate drain. The drain should exit on the out side of the home. Locate the drain on the out side and hook a shop-vac up to it and clean it out.