1999 GMC Sierra Strange hollow metal clicking

I promise to give a big donation if you can solve this mystery. I have had 3 mechanics take a look and nothing. There is a metalic clicking sound coming from under the car. It happens in any gear, moving or stopped, and when stopped, more often then not when the truck is on an incline with the front higher then the rear. It's not the fan blade or rattling from exhaust manifold shields, and it doesn't happen all the time. At first I thought it was something that broke loose inside of one of the cat convertors because it sounds a lot like a marbel slapping around the inside of a hollow tube when I accelerate but we took a listen with a stethescope and found nothing The closes I have gotten hearing wise was near the starter. Any ideas?
Thanks Mark


All right, I owe you like I promised. I'll give a donation as soon as I get paid.
My starter went out on my MOnday and I replaced it Tuesday.
After that, no more clicking
Thanks, you guys are pretty good considering you didn't even see or hear it.
My other? Was about my trans shifting from 1st to 2nd when it's real cold. It takes a long time to shift when it's below freezing so much so that I have to drive around y block a few times to let her warm up so she'll shift before I take her on the Hwy. Once it warms up it shifts fine.