Rocker shims

So I installed Stealth heads a while back on my 400 and wondered if the extra noise was cause they were aluminum. So I pulled a rocker cover while the engine was running and it seemed a lot of noise was coming from 2 rockers, so I pushed one to the side and the noise quieted right down. Is this a geometry problem or is there an easy way to shim a rocker to give it a little clearance? Its not a crazy lift cam, about.505 if I remember right. Thanks for any help.