I suspect the bushings getting dry. Next time you empty the hopper add a few(not too much or it will make a mess) of oil(10w or 3 in 1 red can will do) on the lower bushing. If it gets quieter remove the auger cover and oil the top bushing too. The top cover is silicone to the hopper so be sure to get some hi temp(permatex ultra or rutland) at the local hardware before you remove it. If that doesn't help, Then I'd look at the gearbox. But adding the oil is a cheap solution that should be done periodically to keep them from drying out. Keep us posted!!