periodic rattle at idle

i have noticed recently that at idle after I have been driving, while the truck is either in gear or in park I can hear a periodic rattle coming from under the cab.  I crawled under a was able to pin point noise from around transfer case.  is there some kind of heat shield above transfer case or could the transfer case itself be making the noise?  rattle lasts just a second or two and elapses for a short period before doing it again, also does not make noise consistantly?
 
Thanks
 
136000 mi. 

Not sure on a D2, but I would guess. Also tap the cats with a rubber mallet and make sure the membrane isn't broken...

It will be either the heat shields or a cat that is failing.
If it is a cat, you will need to find out what caused it before you replace it, or it will happen again. Have you had any check engine light issues over the last year, how about tune ups, is it current, leaking head gaskets or oil consumption??