Is this an rpm related ticking or a rough idle rattle? I note you're .mil, any of your buddies got a grudge against you that would know how to set a time bomb? You're not stationed in rattlesnake country, are you?

A rattling heat shield is certainly not uncommon and is easy to check. If it moves it can rattle. This would mostly be related to heat expansion and engine vibration. My experience with loose heat shields is increased rattling when hot. The $.02 fix is to wedge a sheet metal screw between the heat shield and exhaust pipe, but a stainlees steel ring clamp is often more satisfactory.

While we're talking rattles and since you're near the exhaust manifold, there's a flexible aluminum pipe that comes off a heat shroud and heads up and along under the rad to the air filter box. Sometimes this pipe and/or the shroud are taken off during an oil change and may not have been put back on properly. The pipe also often splits at the clamp on the fender. Any of these situations could in theory cause a rattle related to engine vibration. If the flexible pipe is broken you can often stretch the old pipe to fit again if it isn't too brittle (easiest if you remove the whole thing).

If the ticking is related to engine rpm as your note suggests and you've eliminated the valves then you need to decide if it's internal or external. For internal noises, you can always try the stethoscope approach to try to isolate the sound. At your mileage I'd want to check the timing belt, especially if you don't know when it was last replaced.

For external noise, damaged belts could easily cause a soft ticking sound that might go away after warmup. A visual inspection is probably in order. I've often been able to isolate belt noise by spraying with a fine stream of water while they're turning to find when the noise changes.