Car rattles (under car) never heard before, then I start losing engine power

My car has 270, 00 miles. I recently put a lot of work in it, new alternator, new distributor, new AC compressor, full tune up, oil change, new timing cover, timing cover gaskets, new valve cover gaskets, all new o- rings, new water pump, new thermostat. The car has been running like a beast! But the other day I start it up, car rattles (under car) never heard before, then I start losing engine power. The car got harder and harder to accelerate. Then it finally stop accelerating all together. I can rev the engine up too 5000 rpm's, and still no acceleration. Also, there is a loud noise as if I had a race car or a missing exhaust part. Can bad catalytic converter cause car to lose engine power, and stop car from accelerating at all? What is my problem? Any help greatly appreciated.