Effect or Technique

It's probably the guitar's sustain and distortion doing that. My Aria does that when I've got it cranked and hit certain sweet spot notes on the fret board (Usually at natural harmonic points and sometimes at random other places for seemingly no reason). It sounds pretty neat-o. If you want to hear something really neat try playing a doublestop on the second and third string with your finger one fret higher on the third string (Ex. second string 9th fret, third string 10th fret). Now play them and slowly bend up the third string. For some reason which I'm too lazy to research, this creates two different frequencies, on ascending and one decending. The descending frequency is very subtle and you have to listen closely for it. It's really funky sounding, especially if you've got your amp cranked up.