"From what I remember, in the earlier versions, the Stormtrooper turns on his comlink to talk just as the head bumps, so people thought that the "pssht" noise of the comlink was the soundeffect for the Bump."

Correct. This also brings to mind the "noise" that is heard when Obi-wan distracts the two stormtroopers at the tractor beam station. Again, there is a random machine sound effect that is heard near the time of Obi-wan's gesture, but careful analysis shows that the sound effect starts seconds before Obi-wan begins his gesture.

ADigitalMan, I would suggest comparing the two sounds. They are different.