The are supposed to kinda buzz when you have a load on them. If you have no load and it's buzzing, here is what was wrong with mine.
When mine started buzzing and it was a real low buzz, and you really had to listen for it.......it had a blown capacitor. When we opened it up it looked like someone shot one of them with a 22.
Check the volts coming out of it and leave the tester "on" it after it reaches what it is rated for. Mine started climbing and wouldn't stop!
Converter was literally frying the battery.
Check your house battery right away. Good Luck.