From this Sound On Sound article:
https://www.soundonsound....cks-david-bowie-heroes "David likes to do these backing tracks, he gets very enthusiastic about them," Visconti explains, "but we send the band away very quickly and maybe keep a person like Carlos for an extra day or two so that we can double-track some of his parts. With 'Heroes', on the other hand, we built the track over the course of an entire week of careful overdubbing. For instance, Brian brought his EMS Synthi with him, which is a synthesizer built in a briefcase, and it has no real keyboard — it's got a kind of flat, plastic keyboard which Brian very rarely used. He used the joystick a lot, and the oscillator banks, and he would do live dialling — they look like combination-safe rotary knobs on the three oscillator banks. Brian goes down on record as saying that he's a non-musician — he even tried unsuccessfully to have that listed as his occupation on his British passport — and, like David, he thinks very radically and from a completely different space.
"So, after recording the live rhythm section, everyone went home and a week later we came back to this track that was tentatively called 'Heroes', and Brian took out the EMS Synthi and got this shuddering, chattering effect by using oscillator 1 at a very, very low frequency rate — probably five cycles per second — and working the noise filter. He would slowly change the speed or change the intensity with other knobs, and he did that in a couple of passes of the tape, which by now had been edited down to just over six minutes. If you listen to the track now, this shuddering, chattering effect slowly builds up and gets more and more obvious towards the end, and that kind of set the mood.