Sometimes there is an element of that but more often than not sources are from studio recordings, outtakes etc. Some of the sounds on ‘corridor’ are actually fragments from recording in Sweden for the EXIT NORTH project. Or another example is the occasional bass rumble on the track that starts at about 30 minutes (formerly known as STER_01) which is something I had of Mick Karn fumbling around on the bass during an improv session, testing that the bass is switched on, so it’s all rather random, but when you collate these various things together along with environmental noises etc. (and it is very time consuming finding compatible parts, not just throwing anything into the mix) things start to take on a life of their own.
Funny you should mention ‘hum’ though because I was considering the title ‘Corridor Hum’ but when I googled it it showed up as a special effects sound for Doctor Who not too dissimilar to the sounds I might have used on the album, so not a good idea.