Under heavy acceleration, I began to notice a loud noise that sounded like two metal components slapping together. This problem developed quite slowly over about 2,000 miles of driving. I noticed that the vehicle began lacking power and my gas mileage dropped by about 30%.

Took the car to a local Mitsubishi dealer and they were able to reproduce the problem. Turns out that the slapping sound I was hearing was a result of a plugged catalytic converter. Exhaust was actually escaping through an upstream spring-loaded flange which was making the slapping noise.

This would not have a been a huge deal, except that Mitsubishi only sells new catalytic converters as part of the entire mid-section of the exhaust (part alone was $1,133). Once the mid-section of the exhaust was replaced it was like I was driving a brand new car. Problem developed again about 50,000 miles later. Instead of replacing the entire mid-section, I purchased a slide-on converter and installed myself by cutting out the old converter with a reciprocating saw.