Noise on 1996 Chevrolet Blazer

My truck makes a horrible noise coming from underneath in middle of truck. The only noise I can describe it to is a rock tumbler.

Asked by for the 1996 Chevrolet Blazer
I think its the center support bearing located in the middle of your rear driveshaft and I see three different styles available from $69 to $108 depending on which style your blazer came with, about an hours labor to remove and replace. I didn't see your response to bretb's question about moving or stopped as the center support bearing would only make noise while moving, maybe the tork converter is the only thing that would make noise while not moving.
Thank you for your response. I have had a mechanic come to diagnose it. He thinks I just need a whole new engine? But when I start it up the engine sounds fine to me. The sound is like metal hitting metal. Underneath, in the middle and more towards the front.
1 more answer , 3 more comments
My first impression is that one of the catalytic converters is bad.
Is this noise occurring when you are moving or stopped? Or both?
Perhaps the flex plate bolts are loose. I've seen this cause an "engine knock" type noise. This is fairly inexpensive to have checked if you cannot do it yourself.
Nope its not my catalytic converter. Its makes the noise moving and stopped. Its more engine related Im thinking.
Underneath, where the noise is coming from, it has a flat plate covering it.