Valve lifter noise after warming up

[Flynman]

I have a 2006 Chevy 6.0 LS engine in my 56 Bel Air and after warming up and driving for 20 minutes i start to get a lifter/rocker arm tick on the left side. Does not do it when the engine is cold. The rockers just have torque adjustment. The car has sat for a few months before i purchased it this week. Sticky valve lifter?? Any ideas??

Thank you,
Keih R