Many systems are driven by a "black box" which contains a switched-mode power supply. This uses a ferrite transformer (they tend to emit "mechanical" noise) and you are probably hearing either the frequency of oscillation or a sub-harmonic of it. Line transformers on early TV sets were notorious for the circa 20kHz whistle they produced, but only kids, cats and dogs were bothered by it.
In your case it could be the ceramic resonators that you are hearing (resonator being the key word)