Buzzing sound coming from engine?

I have a 1993 eclipse turbo. The guy I bought it from had the timing belt replaced, and 8 of the valves. There is a plastic buzzing sound coming from the engine. It sounds as if you would hold a piece of plastic up against a fan, and let the blades hit it. I noticed that it lost a great amount of power when it started making this noise. I have no idea what it could be.