[sdrothrock]

I also have a cochlear implant; it doesn't allow me to hear ultrasonic frequencies -- in fact, my hearing range is still a subset of "normal."

[mercurywells]

I have one as well, and I've noticed that area proximity detectors (in hallways used to detect presence/motion) do get picked up and come through as loud pops.

[raindear]

There are also electricity poles with a strong EM field that cause a humming sound.

[shepherdjerred]

How loud? These are audible to me without an implant (unless some are audible and some are inaudible)

[jhugo]

You might be hearing a relay, which is itself switched by the proximity sensor.