You could be hearing the normal noise from the transfer case chain. Sounds like that travel.
Go under and use a short piece of heater or garden hose like a stethoscope to locate the source. The T-case chain flops around a little when there's no load on it. Not a problem.
You probably just never noticed it before - or you put new mounts (poly?) in that transmit the noise more.
An awful lot of gears and bearings are turning in neutral - with no load to stop them from rattling.
Neutral, clutch pushed down everything stops turning - no noise. In any gear, clutch up or down there's enough load to stop the rattle. Worry if accelerating it makes noise.