These sounds come from the length of string between the fretted note and the nut. They are actuated by the force of the hammer-ons the same way the as the string length from the fret to the bridge. Obviously, the fret to bridge length is the portion that is "heard" by the pickups and sent to an amp.
If these sounds are very obvious and bothersome, the usual causes are a neck with too much relief, a nut slot that is too high, or a combination of both.
"I taught them everything they know, but not everything I know" - James Brown
Last edited by 20db pad : 02-09-2013 at 08:07 PM.