Even if it’s old, it’s a very good question.
my niece has ears like a bat. she can hear very high frequencies, way above 16kHz. e.g. some switching power supplies or my plasma-arc firelighter.
possible the transmitter frequency is emitted by any parts (wires, inductors) and can be heared…if there were no noise from the alternator, fans etc….
rembering my childhood, where the pilot-tone from UKW-Stereo or PAL-televion was VERY annoying at some cheap devices…