as the transmission crumbles

Not the T/O in this case - I believe he’s talking about the internal rattle most manuals make - that don’t have Dual Mass flywheels that dampen the vibration. I’d say it’s because the input shaft is rotating with the motor with no load, so the synchros and or hubs are free to rattle around. Mine does it, ( and it’s worse the lower the idle is) - as does my ‘12 Volvo, since I installed the single mass flywheel
 
A normal rattle in a manual idling in neutral is from gear lash. That's the shock to the input shaft from the firing of a cylinder. The rattle is the lay shaft gear teeth bouncing against the input shaft gear teeth. Some engines and transmissions are worse that others.
 
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