During some live versions on 'Cymbaline,' there's a pause of several minutes in the middle of the song, where you can hear all sorts of strange noises. This was in fact the Floyd showing off their sound system -- the famous Azimuth Co-ordinator.
In addition to their normal quadraphonic setup, they would (location permitting) also set up speakers on multiple levels -- on balconies and such. They would then, using a joystick control, send sounds up and down stairs, around the audience, and all about the concert hall. This is what you're hearing.