The above .mp3 is a recording of one of my guitars, plugged-into a guitar amp, with the amp turned most of the way up without any guitar-playing. The guitar-pickups are detecting the magnetic pulses that emanate from the SEPTA overhead lines. If you’re within ~300’ of a SEPTA regional rail line, this noise will potentially show up (completely intermittently) on any record players / electric guitars.

The key center-frequencies are:

2.35KHz
4.70KHz
7.10KHz