I just buttoned up the engine from a head swap. I installed a set of milled prc stg 2.5 LS6 heads, OEM gaskets, using allready installed ARP studs. I also added rebuilt stock rockers with the Harland-sharp mod, new 7.350 pushrods, LS6 valley cover/pvc system, Fast 92mm intake, NW 90mm TB, new oem coils, new MSDD wires, new TR6 plugs.
I fired the car up for the 1st time and it started on the 1st turn of the key. I shut it down after 20 sec to check for leaks, none found. I fire it up again and all seems fine, then about 30 sec in I hear a loud ticking sound.
Any thoughts?

 

Are you sure your push rods are the correct length?(Did you check) Are those heads milled? If not Id think those rods are too short

 

First thing to check would be an exhaust leak off the header.