Tranny or Transfer case question

[spectratoad]

I have a 2000 GMC pickup. A couple weeks ago the transfer case (I think but not sure) started whining pretty loud and Saturday on the way home when I would let off the gas and coast I get a loud "chattering" noise. When I get on the gas the chattering stops. The whining is always there though.
I think these xfer cases have a chain, NP203 I think is what I saw when I looked under last.
I am sort of just trouble shooting right now to see where to start.
I thought about putting it up on blocks run it and see if I can tell where it is coming from but I don't want a darwin award. :rollside:

[Sleek-Jet]

Could be a chain... how many miles???

[spectratoad]

74,000. Shouldn't be worn out yet. I bought it from HER454 and I know she took good care of it and I don't think it was ever off road much if at all.