Yep...I've experienced the same noise...sounds like the belts are really tight...but like Sleeve84028 said...I think after a while it loosens up and the noise goes away.
Does anyone get a noise like a turbo whistling from their KJ?? I've always had this neat little whistling and my friends are always puzzled when they hear it. I'm thinking its the air intake off the airbox....
Me and Video Guy have been experiencing negative temps here in Indiana.....

I have been keeping my KJ and my mom's Pontiac Torrent in the garage with a small space heater warming the garage for easy starting. Seems to start well on the cold mornings....
On saturday I took my girlfriend to Logan's Roadhouse for some dinner. After we got out I started the KJ and as I went to back up I heard a LOUD crunching and moaning while backing up. I didn't even go a foot before I stopped and frantically put it in Park and then putting it in drive again thinking it was the tranny. I then went to back up again...same noise....so I got out and looked to see if there was an ice chunk or something stuck somewhere in the rear/front of the jeep....nothing. So now I'm really panicing and trying to figure out whats going on. So I get back in and I for some reason think to put the jeep in Neutral. It won't roll...so I think for a minute as the KJ warms...I then think about the snow/ice gathering in my wheels and the rear drums....after a few minutes the jeep starting rolling backwards and the noise went away. The drums must have froze up from the ice/slush gathering in the wheels....and it provided a nice scare!

I was definitly freaking for a minute....but after some quick thinking I quickly knew what the problem was.