As for a test, unplug it.

As to can they hear something you can't yup. Ignoring sounds that might or might not be comming out of the actual speakers, there is an effect called the piezoelectric effect. Ceramic capacitors are the worst for this. When they are physically flexed they induce a voltage across them. When a voltage is induced across them they flex. If the frequency is right it will make an auditable sound.

A lot of electronics use ceramic caps and are apply a frequency near the high end of human hearing. Most people can hear it. I'm one of the "lucky" ones that can. A lot of electronics emit a high pitch ringing noise that most people dont seem to hear, yet drive me nuts. Starting to question my sanity here.