Noisy in reverse

[Edthehorse]

I have a 73 duster 340 4 speed standard. Just bought it. I'm.getting a deep whirring noise in reverse gear but not In any forward gears. Any ideas what's going on?see video of sound. I jacked rear end off ground and put it in reverse. Doesn't do it with clutch pedal to the floor. Only when in reverse and moving

 

[Phreakish]

Reverse is a straight cut gear. It will whine a bit. Pretty normal.
On jakcstands and leaning out it'll sound worse since the noise bounces off the ground and can sound amplified.

 

[Edthehorse]

It sounds just as bad on the ground backing up

 

[TrailBeast]

It sounds just as bad on the ground backing up

I would have to suspect the reverse gear and pin assembly for wear.
Sounds like too big of a gap between the meshing gear teeth.