Sounds like a cable.
 

My right speaker makes noise even with the cable unplugged.  It's not coming from the monitor either.  The other speaker is closer to the computer but no noise.  It does sound like digital noise it's picking up.  Still troubleshooting.
 
Jim

I traced the noise in my speaker to a network switch.  Moved the switch under the desk and the noise is now gone.
 
Jim

Regardless of where the buzzing was coming from, you should use good balanced connection cables from your interface to your mons.

 

gustabo
Regardless of where the buzzing was coming from, you should use good balanced connection cables from your interface to your mons.

+1
 
I just solved similar problem by using XLS balanced cables. If your interface does not have XLR outputs, use XLR to TRS extension.

I do have balanced cables.  It was still buzzing without the cables plugged in so the issue wasn't with the cables.  It appears the switch puts out a decent amount of RF. 
 
I've been using balanced cables for years. On the other side of the wall is the main power coming into the house and I use to pickup noise when the AC units would turn on.  The balanced cables fixed that issue.
 
Jim