2000 Mitsubishi Eclipse Loss of power, change in exhaust so

Sound of exhaust changed one day from throaty to more of a chainsaw type sound. CEL light came on. Car seemed to have less power. Mitsu dealer checked, put new plugs, wires, rotor etc. On, reflashed computer, didn't seem to think exhaust sound was issue. 2 days later CEL came on again. Took car back to dealer. Ran compression test. Said left bank was a little low but not bad. Flashed computer again. Said car was OK to drive. 1 day later CEL on again. I took car to local mechanic. He found an issue with Map to PCM wiring. Fixed it. Exhaust went back to sounding normal. Car drove 2 weeks before exhaust sound changed to chain saw again and CEL came on again. Do I need to rebuid the engine, or just swap out the engine and computer? Could there be an issue with the Catayltic converter ( dealer said CAT was OK - but I can't figure out why the change in exhaust sound)