Noise when cold

My 1999 4D 4WD Blazer LS (50k mi.) has started making a rattling or ticking sound. The sound seems to come from near the transmission or transfer case (under the feet of the front passenger), happens only when in gear, varies with RPM, and disappears after 5-10 miles driving. Oil checked full; transmission fluid checked full, but is due for change.

What could this be?

Thanks, Brynturk

when the truck is stone cold, get under your truck and smack the cat convertor with the palm of your hand and see if it rattles.