2008 Tacoma: Front end rattling...any help?

I've got a 2008 Toyota Tacoma Double-Cab, 4x4, TRD Sport. I've had it about a year and have about 14K miles on it so far. I was driving today and, at one point when I came to a stop (but still had it in drive), something in the front-end starting kind of "rattling" or "clicking"...I could feel it a little bit with my feet on the drivers-side floorboard. I tried it in several different gears and the clicking would persist in all gears (except Neutral and Park). It's a snowy day, so I had been driving around most of the day in 4WD. At the time when this happened, I had switched it to 2WD a few minutes earlier.

My question is...has anyone else experienced anything like this? Or have any idea what might be causing this? I don't think that it has anything to do with the 4WD because I was able to reproduce the "rattle" after driving around in 2WD for a while, but I could be wrong.

Any ideas? Any help is MUCH appreciated. Thanks!

AE