As you are rotating the gear box have you cleaned the gears and checked them for any damage.
I use a stiff brush to clean out the teeth and the under a bright light check the teeth for any damage, once you are satisfied all is good wash all the bearing in WD40 and blow off then soak the bearings in Triflow and then allow to drain. Check each bearing for rough spots and if then all is good rebuild the gear box dry installing each gear one at a time, so that any tight spots can be found, once satisfied it is ok grease up and reassemble.
it is a long job but I have found it works, and some thing else to think about is that this a gear box in plastic cases and is never going to be a free as a watch but can be very close with patients.
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