Does this noise go away above ~2000 rpms?
The reason I ask is I also noticed a noise coming from the same area (when it was still a V6) as you describe. I would hear it in neutral stopped at a light. IF I pushed the clutch in, the noise would stop as the gears in the trans spun down and stopped. I would also hear it (most noticable in 4th gear) under 2000 rpms. I thought it was just miles on my transmission and maybe a bearing going bad. It didn't seem to hurt anything, so I kept on driving the car.
I changed the trans fluid to Royal purple, and I noticed NO difference.
Fast forward to now, and my Brand NEW tremec has the same noise. Stopped idling in neutral OR driving under 2000 rpms. It is gear rattle. A thicker trans fluid may quiet it down, but Tremec says "Dexron III" fluid only.
Back in the toploader days, it was Gear Oil in the gearbox. LOL