One of my valvues is making a horrible noise, how do I adjust?!

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I just bought a 2001 audi tt quattro, an the previous owners had it taken to a mechanic to have the cover gaskets replaced. Apparently from the knocking noise, it is one of the valves. I cannot find anywhere on the internet on how to adjust the valve. I have had BMW's so I am new to working on the engine of an audi. Any help? Thanks.