lifter perhaps? check the oil, i think sometimes it can be remedied by that. oh and make sure it's not exhaust. isnt the trick to put a screwdriver on the valve cover to see if its valve chatter?
Most likely you have either a collapsed lifter and/or a bent pushrod. Hopefully no broken valvesprings. If you're lucky it's just a pushrod and you won't have to pull the lower intake.