It's buzzing because there is a small set screw that keeps the drive belt from the motor turning the shaft that connects to the chain via a cog(similar to a cog on a bicycle)that moves the chain that opens the door and that set screw is loose and needs to be tightened using an allen wrench.

I paid $95 to have a repairman show me the issue. I've had to retighten it twice. The 2nd time I really torqued the set screw and it's held for about a year so far. Eventually, since the set screw is made from tool steel and the shaft is a softer steel, I'll likely have to drill a hole in the shaft and replace with a cotter pin. Luck.