Start looking for a friend that is more experienced with sleds to help eliminate some of the basic/obvious things. There are a lot of things that will cause a sled to run poorly and you can chase your tail in circles for a long time and miss something very basic/obvious to someone with more experience working on sleds.
Belts, clutching, running or intermittent running on one cylinder,frozen idlerwheels or other driveline bearings,dirty carb,plugged exhaust(dang mice),too tall of a windsheild(jk), etc.
The humming is probably just common track noise, the fresh snow will dampen the sound and give better slide lube.
Good luck, Sno