Umm, me thinks you have something loose that's rattling against the housing.
The give away is it's worse in gear sitting still. Just like a bad mount on the eng/trans can cause enough of a shift for the exhaust to vibrate against something.
A bad mount will transfer vibrations though the frame to the body, and get worse when loaded.
Kicking the a/c-heater blower on and it not changing the sound pretty much eliminates something caught up in the blower housing.
That housing will act like an echo chamber so something banging against it out in the engine compartment will sound into the passenger compartment.
If you have a helper, have them hold the jeep in gear at idle, open the hood and look for something rattling. Also watch the eng/trans for excessive movement when dropped into gear, a sure sign of a busted mount.
No doubt stuff can get into he blower housing, rodents love the nice dry spot, and if you are down south snakes too. But I don't think that's your problem.
Funny I just remembered, a year ago one of my Insurance adjusters came in the shop and told me my Colorado was mewing when he walked past it.
We walked out and sure enough it was meowing. We ended up pulling the whole grill out to find a kitten had crawled up and gotten wedged under the drivers side headlamp, and couldn't get back out.
It ended up going to a good home home with the insurance adjuster.