2002 Pontiac Sunfire

My 2002 Sunfire had a slight (only audible on occasion and only in R or 1st gear) knocking noise when I bought it 2 months ago, that I assumed was an exhaust leak. However, this evening while driving up a hill on the highway my car started to lose power, and would not pick up speed, even with my foot buried to the floor in 5th gear. Once I made it to the top of the hill everything seemed to be fine. About 20 minutes, after dropping a friend off and while backing out of his driveway I noticed that my engine was knocking very loudly, almost a metal on metal sound as soon as my RPM exceeded 2000 (I'm going to assume the engine was knocking before hand but neither of us heard anything because we had the radio on and up. The knocking is similar to the one I assumed to be an exhaust leak, only stronger, louder and MUCH more noticeable. The sound is definitely not an exhaust leak and isn't caused by any visible vibrations upon quick inspection. The sound seems to be coming from within my engine, could this be a camshaft problem? Or if not, any insight into what this could be? Thanks, Justin