Spark Knock????????

Hey guys, I need a question answered if anyone can. I have a 99 tiburon with a 5 speed manual transmission. 2.0 and when i shift from 1st to 2nd and 2nd to 3rd i hear a knocking noise. do you know what this is???? I dont know what to think, is this normal, do i need a knock sensor??? is it my spark plugs???? a timing belt??? please help if you have any idea. thanks