I have a corolla with a 4AF engine (carburator). there is a constant tapping noise at idle. When I rev the engine it dies down abit. What could this be???? i thought it was the lifters so i've changed intake side but the nosie is still there. someone had told me after that there speacial way of removing the cam and placing it back in. Please could someone help as it is fricking pissing me off? Thanks.