You must have the power supply connected as those are "powered" speakers (have a built in power amplifier).
The hum may be a loose or partially inserted plug. Check the plugin at the PC. If it's only partially inserted or if it's a "bad" connection you can get hum.
Also, if the power supply goes back you could get hum - or there can be an internal problem in the speaker amplifier that can cause it.
Unplug the speakers from the PC and then power it on and see if there is hum. If there is, the problem is either the power supply or an internal electronics problem in the speakers. If there is no hum, before you plug the speakers back in connect headphones to the front panel headphone jack and see if there is hum there. If there is none, it's most likely a bad connection from the speakers to the PC's speaker jack. In that case unplugging and plugging the speaker plug back into the PC - making sure it's fully inserted may fix it. If not then most likely it's a bad cable between the PC and the speakers.
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