Actually - what you see is the noise of many things going on on your system - it mostly happens on Harddisk Accesses, Sound-card playing music, Strong Network-Transfers etc.

• It has something to do with the Powerunit/Mass connection and the connection between the Screen and the Onboard graphic card
• Once you make sure the Case and the Mass of your Board is connected to the Generall Mass (in Germany it's called "Erdung"), this noise apparitions go away on your Screen
• It can also have something to do with the Powerunit not beeing strong enough - so you can try out a stronger Powerunit to see if it solves the problem