1999 Chevy Z71 ExtCab SWB metallic noise when accelerating

I have 1999 Z71 Ext Cab SWB with a 5.3 ltr AT. When parked or in drive and rev the engine either slow or quick, there seems to be an odd metallic sound coming from under the truck near the transmission on the passenger side. It is not a constant noise and is only intermittant (comes and goes). It kinda sounds like a radiator flex fan just before a blade broke, or when the blade would strike the fan shroud.
Thought it was the UJoints so I had all replaced and the noise still comes and goes. I have checked from head to tail and can't seem to locate the culprit. The only thing I can think of would maybe the flex plate. Would it make any noise like that if it was cracked or broken? No starter probs to date so I am lost. This has been driving me nuts and I can't seem to figure it out. Any suggestions from anyone would help.
BRIAN
:banghead:

Check the exhaust. Make sure it's not vibrating against a heat shield. Hit the exhaust pipe with a rubber hammer (with the engine off), if something inside is rattling, you will hear it.

The metallic noise sounds like the heat shield being struck or vibrating but have checked all of them. They are not loose or within and inch of the exhaust components. I will try your suggestion tonight and let you know. Thank you very much.