Engine-transmission rattle

You neeed to locate the source of the noise to understand what could be wrong.
Depending on the type of noise and source, it could be anything from a bad component to a loose bolt/shield somewhere.

Do you have issues with gears up and down shifting and did yu check for trouble codes.

Trans shifts perfect. Noise is at approx 600-800 rpm only. Looked everywhere for loose parts-shields.Noise centralised to bell housing or trans pan.No inspection shield to look at flex plate or bolts. Why it is in this rpm range is baffling otherwise engine and trans performance are great.Would code get set& not CE light?

Some codes don't trigger the MIL.

Noise at low rpms are usually due to something vibrating. Does noise occurs when in reverse too?

There is an access plug for removing the torque converter bolts. I do not have the location but if you remove the plug, you should be able to see the flex plate and torque converter bolts.

Noise does occur when in reverse but goes away as soon as brake released and engine accelerates.I will check for access plug and see what I can see.

Any possibility of engine mounts being bad resulting in the vibrationg noise?

Will have to look at that but there seems to be no excessive engine movement and noise really seems to be from trans-bellhousing area. Almost like rod knock. Saw another site with balancer shaft issues for 4.3 engine. Hate to say but not sure what that is. Chev put out service bulletin but I do not think my year was included.Will look more when time permits after work. Will get back to you.