somethings wrong

My friend has a 91 jetta 1.8. It has a ticking noise from the top end when it heats up. Then as soon as it came it goes away. I pulled the valve cover off and the single cam looks great. good oil pressure and no heating up. I dont know if I shim the lifters or what to do. the noise is definitely coming from under the valve cover. When i had an 80 rabbit I saw my mechanic use a tool to put a shim? under the cam. Am I far off? Thanks for input. the car has 100.000 miles.

That car uses hydraulic lifters. When they get weak they will start making a ticking noise. They probably will need to be replaced, but you can try a different weight oil (higher viscosity) & see if that helps.