Front driveline noise in 2wd...

[mattging94]

So I've started noticing a sort of ringing/clicking noise coming from somewhere in my front driveline. I don't hear it until I'm above about 20 mph, and it's somewhat faint, unless I'm driving next to a concrete barrier then I can hear it pretty well. The weird part is the noise goes away if I put the truck in 4x4.
I jacked up one front tire off the ground and started spinning stuff around under there, and the closest thing I could hear to what I hear when I'm driving was coming from the tulip joint thing where the front driveshaft hooks to the t-case - when I reverse the direction I'm spinning the tire I hear a similar ringing click. Seems odd that the front driveshaft would be running in-and-out of it's slack while driving down the road at steady speed though, right? Anybody have any knowledge about what may be going on here, and if it's something I should be concerned about?

P.S. I first noticed this noise after driving the truck on sand dunes, so I'm thinking the extra load on the driveline from driving in sand is what created whatever condition I'm hearing