1999 Chevy Suburban Intermittent valve rattling

I have an intermittent valve rattle in my Suburban. It usually will only present it's self when the motor started and has been cold for several hours. Most of the time, it rattles for up to one minute, usually less. Oil is up to spec, changes every 3k with new filter. It may go weeks with no trouble, then will rattle every day for a week then quit. Gas milage has not changed, spark plugs show clean burn, good plug wires, distibutor cap and rotor in good condition. This has been intermittent over the last two years.