High RPM chattering..

I've noticed my engine has begun to have a noise like metal vibrating against metal in high rpms 2,000 and up. It sounds like the engine is going to fall apart but there isnt any symptoms besides that... atleast not yet... the intake valves at hydrolic and the exhaust were adjusted a few months back. its a 90 legend AT coupe... any ideas? i hear nothing at idle or low rpms.