I have also heard rattling noises when idling too slow in gear that people smarter than me have said are coming from the damper plate, which is attached to the engine flywheel.I have run a 5.7 liter Competition Ski with a Borg Warner velvet drive (fnr) behind a Walter V-Drive for the last 26 years and have fought belt squeal a lot, going through several serpentine belts, but your recording does not sound like belt squeal, although recordings are tough to analyze. Check the gearbox fluid level (Dexron IIl ATF?) dip stick while hot and try to get close to the engine and gearbox to isolate the source of the sound. How many hours on the whole set up? Any reason to suspect shaft alignment?
That would be my thought based of the very limited video. Would try and listen with a mechanics stethoscope on flywheel cover to see if that can be confirmed. Would also make sense it is dampner plate if noise occurs under heavy load.I have also heard rattling noises when idling too slow in gear that people smarter than me have said are coming from the damper plate, which is attached to the engine flywheel.