The noise is from the last gears in the drive chain. They are on the idler shaft driving the main gears on the spindle. The spindle itself is tight, however the gears slide on the spindle. The idler shaft drive gear is a dual gear, that is a single piece with a gear at each end, the speed is dependant position of the gears on the spindle.Inspecting large gears for wear is a pretty straightforward visual inspection. Straight cut gears are inherently noisy regardless.
I'd look at bearings first, though. Unless it's been heroically abused with no oil, it's far more likely it needs bearing attention than replacement gears. Worn bearings can make gear trains far noisier if they don't hold them in perfect mesh too.
That to me means there is a bushing on the idler shaft and potential wear on the splines on the spindle shaft. I will try to measure those.