2003 Suburban Z71 transfer case rattle
 

My 2003 Suburban has a rattle that I believe is coming from the transfer case.
Transmission is good, no slipping no leaks, no shifting problems. Transfer case will engage in 4h and 4l. Noise is only present while driving in 2h. In Texas, so I rarely use 4x4. 140,000 miles. Is there a good way to diagnose or is this a job for a pro? Thanks for any info/help.