I spoke with Tom Morello and he told me how to make that “‘screechy scratch’ sound” on the guitar!
“Turn the volume of one pick up to 10 and of the other pick up to 0. When you switch between pick ups, it acts as a killswitch, since one is off and one is on. Once you get that far, rub the palm of your hand along the strings, which is abrasive and makes a ‘white noise’ wash. Then use the toggle switch, which turns the white noise wash on and off… which sounds very much like a turntable scratch.”
Read the rest of my Tom Morello interview over at Noisecreep.