Cams are not constantly touching. When the valves are closed, there's a very thin space between the back of the lobe and the shim. This is what you're trying to achieve when/if selecting replacement shims.
Canadian - You sure it's not fuel injector noise? If you want to try a variant on the "stick test" you can achieve similar results with a piece of vacuum hose and a suitably sized bolt stuffed into the end of it. Touch the bolt to the solid object you're listening to (fuel rail or injector, for instance) and stick the other end of the hose in your ear.
Just the hose without the bolt makes for a great vacuum leak tester, as it picks up hissing air very easily.