hard to tell the humming sound is caused by the synchro, since we overhaul,normally renew all the old parts, might be other stuff make it sounds like that....
easily said, synchro of our gear function to match our gear speed with our engine speed.... 1 small example, like when u using 3rd gear, ur current gear speed will be same as the engine speed, however, when u change to 4th gear which is diff in speed, u nid something to match the speed of the gear and the engine in order to shift smoothly... synchro function as the speed matcher.... that is why when ur synchro out of order, tend to have grinding sound like "grok" when u shifting especially high rpm when the engine speed is high....... but even synchro is out, u still can engaged to the particular gear in low or high rpm but just that the grinding sound is scary and in long term, scare gearbox will got problems...
easily said, synchro of our gear function to match our gear speed with our engine speed.... 1 small example, like when u using 3rd gear, ur current gear speed will be same as the engine speed, however, when u change to 4th gear which is diff in speed, u nid something to match the speed of the gear and the engine in order to shift smoothly... synchro function as the speed matcher.... that is why when ur synchro out of order, tend to have grinding sound like "grok" when u shifting especially high rpm when the engine speed is high....... but even synchro is out, u still can engaged to the particular gear in low or high rpm but just that the grinding sound is scary and in long term, scare gearbox will got problems...


..... maybe that is the reason u guys din see the car long time....