[florianz]

when you have a closer look, you probably see that it's not straight cut, it's easier to see on the pinion. afaik hobao has only these diffs in the st, but I can be wrong.

maybe just the bearings are full of sand etc, because you turned it without load and you still got that scary sound. sand can get into the diff too, when the screws holding the diff on the chassis are a bit loose.

brushless causes a lot of wear, so probably you only need some more shims. that's normal and should be done after a few batteries. after it's "shimmed" (always both, diff and pinion), it must run smooth, but not loose.
I installed a new diff yesterday, as the old one was completely worn out, shims, new bearings didn't help.
good luck
flo