DIAGNOSIS This is usually a simple mechanical resonance, not a breakdown. The compressor and fans are mounted on rubber dampeners, but if the fridge isn't level or is touching something, the whole cabinet acts like a speaker box.
FIX 1) The number one fix: level the fridge. Use a small level on top. Adjust the two front feet by twisting them clockwise to raise or counter-clockwise to lower. Get it perfectly level side-to-side and front-to-back. 2) Pull it away from the wall. Make sure no part of the cabinet or door is in contact with the wall or nearby furniture. 3) Check what's on top. If you have anything sitting on top of the unit (boxes, another appliance), remove it. 4) Make sure the shelves inside aren't vibrating against the side walls. Give them a slight push back to ensure they're seated fully.
IF THAT DIDN'T WORK If the loud hum is coming distinctly from the back lower area and is accompanied by a 'rattling' sound, the issue could be the compressor mounts have come loose or the fan blade on the condenser fan (if it has one) is hitting something. This requires unplugging the unit and carefully inspecting the back panel, which might be a warranty service call.
HEADS UP Some vibration is normal, especially when the compressor first starts. But a sudden increase in noise is almost always a 'contact' issue—the fridge touching something it shouldn't. Isolating it from its surroundings solves 90% of these noise complaints.