With it changing with motor temp, that does sound more like an exhaust leak. The 3.6 isn't particularly know for exhaust leak issues, though. I would suggest getting a cheap mechanics stethoscope from Harbor Freight and making sure you don't hear a tick when you put it on the valve covers.

 

The dreaded tick, doesn’t go away. Because it is caused by worn out parts. Yours sounds like an exhaust leak.

 

A lifter tick won't go away with temp, so you can safely rule that out. Exhaust noise is normally worse on cold, then goes away as it warms up. However, the 3.6 only has 4 exhaust "manifold" bolts on each side so it's not likely to be that either, although it's worth a check. One thing it may be is piston slap, I've seen rare cases of 3.6's have piston slap at certain temps if the cylinder bore warms up/expands faster than the piston itself. My personal '17 has done it on rare occasion, doing exactly what you said. It only happens after it warm up to normal operating temp, then is parked/cooled down to 100-150 coolant temp. I can hear it "flutter" on rev down below 1-2k RPM's until the coolant temp hits operating spec again.