Hi,
I had the SM420 rebuilt by a real shop and put it back in along with a new clutch and throwout bearing a few months ago.

Recently I have noticed loud and getting louder, noises only when off the gas decelerating.

It's not the clutch or throwout bearing, and the transmission has just been rebuilt, so I'm thinking U-joints?
Or, I hope it's not the transmission.
Everything is fine while accelerating, or even just keeping a little forward pressure on the driveline.

Any suggestions where to start checking?

Thanks,