I've got the same issue on my TH (3/15 build date). I've posted in that other thread you linked to, but it would be nice to start one dedicated to this. I plan on waiting until my first dealer oil change and see what they say. The sound (to me) is more annoying than worrisome.
Here's my take on it:
It sound and feels like a weak rumble strip. It is barley noticeable on coarse pavement as it is drowned out by tire noise. Above about 40 mph it is either gone or covered up by other noises. It does not matter if I've been driving for 5 minutes or 5 hours.
I can switch it on/off at will. It's only present in auto-shift and auto-4wd. If I lock 4wd or change to manual shift mode it goes away completely, or will never occur if left in those modes. If I leave it in auto and keep a steady throttle at 25-35 mph the noise/vibration will "switch on" after about 10 seconds. I'm sure I could time it's start with a stopwatch as it seems 100% repeatable on my normally traveled routes.
If it's making the noise/vibration, it will switch off if I either: shift to the manual-mode transmission gate, or lock the 4wd. Within 10 seconds of switching either back to auto the noise/vibration returns. Like clockwork - it switches on and off very abruptly. It is definitely something in the system switching something on/off.
If it is missing in 4 lock it may be something to do with the rear axle disconnect. A grinding noise can never be good.