One of my valvues is making a horrible noise, how do I adjust?! on 2001 Audi TT Quattro

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.

Asked by for the 2001 Audi TT Quattro
not adjustable. you need to remove the covers and inspect for damage

I am pretty sure it is a clyinder valve. If not adjustable, how do I fix?