When recording old analog synthesizers (different brands and models) I'm getting interference in the audio. This started suddenly about two years ago (I thought the synth I was using was malfunctioning, had it serviced, and even bought another identical one, only to find the problem persisting.)
I'm used to old synths picking up a lot of 50Hz (and multiples) mains hum, but this signal is exactly 1000 Hz and all multiples, up to above 20kHz. It's about 60dB below the signal level and 12dB above the noise floor (see image). It's not there if I unplug the synth, so it's not being picked up by the cable or my audio interface.
I'm guessing some other appliance is creating this interference, but when it started I hadn't bought any new audio or computer equipment or household appliance. It could be something my neighbours are using, but I'd like to have some idea of what to look for.
So, what kind of equipment or phenomenon could be causing interference at exactly 1000 Hz and multiples? Is this a typical problem? It seems like a strange frequency for any kind of wifi or other communications signal.
440Hz note with 1000Hz interference indicated in yellow
(In case this is relevant, the synths used are two different Korg MS-20's, a Korg MS-10 and a Yamaha CS-15, all between 35 and 40 years old; one of the MS-20's was recently serviced and had parts like capacitors replaced. Service manuals with circuit diagrams and part lists for the Korg MS-20 and MS-10 and the Yamaha CS-15 can be found online.)