Strange noise when speaker are off

[Dudo]

Hi all, I just bought a klipsch promedia 2.1. When I power it off, a strange and repetitive noise can be heard, I've made a video here

This noise can be heard even if the computer is power off and the green jack is detached. I've tried to plug it in another source, without success.

 

Any help will be really appreciated.

 

Apologize for my bad English,

 

Regards,

Dario