You know the classic moment where the Stormtrooper smacks his head on the pneumatic door in ANH? Has there always been an audible "bonk" when that happened? Note that prior to today I had never heard or seen the films in this quality, so if it's always been there I simply might not have noticed it. But when I saw and heard that today, I damn near pissed myself with laughter.

It's new.

I guess you could call it an audio "enhancement".

I've always heard the sound in every version and interpreted it as the talking trooper's 'tck' sound that they all have before and after they speak.