engine has a knocking sound?

Ok after 2 ppl flaking on me.. i found a dude with a h22 that wants to trade me for my greddy evo2 exhaust....but he told me straight up that the engine had a knock sound.first of all is it a good trade my greddy exhaust for his complete h22 swap that knock? Whats the most likely part that knocks( piston,crank,etc..