Diagnosis
That intermittent loud buzz is almost certainly the evaporator fan motor. It's located behind the back panel in the freezer. When the bearings wear out or ice builds up on the fan blade, it makes that distinctive racket.
How to Fix It
You'll need to access the fan. 1) Unplug the fridge. 2) Remove all items from the freezer and take out any shelves/drawers. 3) The back wall is a plastic panel held by a few screws. Remove them and carefully pull the panel off. 4) You'll see the fan. Check for any ice buildup on the blade or housing. If it's iced up, you have a separate defrost problem. If it's clear, try spinning the blade by hand—it should spin smoothly and quietly. If it grinds or is stiff, the motor needs replacing.
If That Didn't Work
If the fan seems fine, the noise could be coming from the compressor area at the bottom back of the fridge. A buzzing from there could be loose refrigerant lines vibrating against the cabinet. You can sometimes muffle this by carefully sliding a piece of foam pipe insulation between the line and the cabinet wall.
Heads Up
Be very gentle with the evaporator coils (the radiator-looking thing) behind that back panel. Bending the fins can hurt cooling performance.