Why are you plugging a cable into the speakers without having the other end connected to anything?
That's the very definition of 'make a hum noise' - a game played by every guitarist who ever lived, at some point in their career.
It will get worse if you hold any single element of the plug at the other end & get better if you hold both elements with damp fingers, closing the circuit. If it doesn't stop when you do that, the cable or the speaker's socket is broken.
It has nothing to do with your PSU at all.