If you have not yet figured out the problem, look between your spring and your frame in the front. It might be that when your spring compresses, it flexes to the side, toward your frame. once it starts to wear the paint off, it creaks just like you explained. I run into this on occasion with mine. It sounds just terrible, but its fairly easy to fix once you know where it is coming from. someone could put a piece of nylon on the surface of the frame to act as lubrication between the frame and spring.
Just a possibility.
Looking for a money tree.