I just went through this two weeks ago and it was the header gasket that went bad. I was sold that it was a top end/lifter issue, but paid to have the header gasket replaced and that got rid of the noise. Just another victim of aftermarket gaskets for longtube headers going bad. In the end it was $21 in parts and 7 hours in labor to have the gasket replaced.
2006 Jeep GCSRT-8
Mods:Mopar CAI-SW Long Tube Headers-High flow mid-pipes-Corsa/Mopar Exhaust-Diablo Trinity-Custom Tune-FRI Little Boy +4 Camshaft-Mopar TCM-Innovate LC-1
1/8 mile PB of 7.959 seconds at 85.85 mph