chucky

I have a five spead and noticed after it gos past 3000 rpm and then I put the clutch in theres a wierd anlmost knocking noise. Almost like something is lose in the engine. Its sound kind of like its back firing in the engine. But sometimes I can feel it on the gas pedal. Wht the hells gong on here? It seems to only do it when the car is hot.
Im almost starting to regret putting it on. I dont want to screw my engine up. WWhat do you guys think???