Did you try going around your house and turning everything off to see if it stops? If you are in an apartment it could be your neighbors stuff.
Does the variac hum with nothing plugged into it? If it only hums with something plugged into it then something might be loose in the Variac, but I still think noise on the line is the most likely culprit.
Does the variac hum with nothing plugged into it? If it only hums with something plugged into it then something might be loose in the Variac, but I still think noise on the line is the most likely culprit.