To make this sound I increased the gain on some 60Hz buzz that I grabbed out of a bass guitar recording. There’s no bass, just the hum of electrical interference. A lot of boosting was necessary to get an adequate level.

Afterward I time stretched the waveform by varying amounts at consistent time intervals and manged to create this rhythmic pattern. A touch of chorus gave it some stereo spread and a short delay put it into space.

Time Stretched 60 Cycle Hum