It is a bit unusual that the tick is sporadic/sometimes and when the motor is up to full operating temperature. As it is someone common for these motors to be a bit noisy on cold start ups and then quiet down.
There is likely 3 scenarios that could cause a ticking sound sporadically. The most likely would be sticky lifters or dirty lifters that might have trouble holding oil pressure. Auto-Rx is very effective in cleaning out lifters/guides, etc. The second scenario would be an exhaust leak, that may not be heard when the motor is cold, but as heat expands the metal the leak gets more pronounced. There have been reports of exhaust manifold bolts loosening up on these units. And usually back close to the firewall. Or lastly the fuel injectors may be dirty or not getting full fuel pressure/flow through the rail(possibly dirty fuel filters). Typically you can only hear injector noise at idle and is a lighter ticking sound compared to a valve or lifter issue.
I assume that the motor runs well, with no codes or check engine lights. I would suggest running the 2 bottle Fast Track application to clean the valve train thoroughly based on the mileage. You could always through a bottle of a good injector cleaner, such as Techron in a tank of fuel to see if that helps. And use a sheet of paper to place close to the exhaust manifolds to see if a leak is present. Or just look to see if a bolt has backed off.
