Just to verify the ground loop, try unplugging the AC/DC charging adapter from your laptop and run the laptop on the internal battery. I suspect the hum will go away.
The ground loop is likely a voltage differential between your incoming cable-TV box (do you have one?) or satellite receiver.
Try connecting the video cable from the laptop to one of the video inputs on the pre-pro. If the hum goes away, great. If not, unplug the incoming cable to your cable TV box. If the hum goes away, then you may have to install a ground isolator on the incoming cable to the cable-TV box such as this:http://axiomaudio.com/groundisolator.html
Note: this can't be used on a satellite receiver, only regular cable-TV.
What is the AC/DC adapter for your laptop plugged into? Try connecting it to the same power strip or circuit that supplies the TV power.
Some combination of these should work.