ok, what does it actually do, increase the capacity or inductivity of ?, so the frequency is shifted to somewhere we don't here? Changing some other electrical propertie to avoid to osicate at such frequency?
some switching power supply inside the bolt does not have enough load and the frequency/duty cycle makes the coil resonate into the audible spectrum, probably
hence also why the noise was changing along the blinking LED program that came with the arduino as a test
this type of switching regulators typically runs in the frequency range of 15 KHz all the way up to 100 KHz. I am not sure exactly the type used in BOLT, but I suspect the OP's hearing is extra sharp to be able to sensitive to the ultrasound range.
