Since that edit was from a long time ago I don't remember exactly, but I think the sound you're hearing is a repetition of their footsteps as they begin to run.

If I recall correctly, it was the only way it could be arranged so that there was no obvious shift in the rhythm of the background noise, which had a kind of regular pulsing sound. It would have been easier with discrete channels, since it could presumably be cut from the centre channel only without affecting the rest of the mix, but we are limited to stereo. Although even some 5.1 mixes have a bit of dialogue overlap into the left and right channels, so who knows.

Mallwalker posted in d_j's thread about some errors in the first copy of the '93 pcm that Belbucus later corrected. I've never heard any of those, so I don't know whether it is worth making another version to fix them or not. The picture looks like a very small pop or click in the left channel over the course of 6 samples, or 1/8 of a millisecond. I suppose if I did end up correcting them, I'd put the spit sound back in, since it 'should' be there for authenticity's sake, even if it is a mistake. I have definitely heard at least one glitch in the '93 mix that occurs during the main title (the beginning of the second phrase of music, corresponding to the second paragraph of text in the pre-ANH crawl), which is also a clicking sound in the left channel. But since this is also heard in the GOUT, it appears to have always been this way. Whether the 70mm printmaster itself was a bit damaged or if it was simply an error in the transfer, I don't know.

There are also two such clicks in ESB--one as the probe droid emerges after landing on Hoth, the other as the Falcon detaches from the Avenger. Both appear in the GOUT as well; no idea what caused them.