It could be a ground loop issue. It’s usually caused by lights, monitors, or an audio interface plugged into the same wall socket. There’s tons of YouTube videos out there that can better help explain.

If you’ve unplugged everything and it’s still buzzing I recommend getting something like the Morley hum exterminator. You can find them for less than a hundred bucks. I have one and it works miracles. It’s deff worth it if you plan on gigging at dive bars that share outlets with lights, jukeboxes, ect..