Ok, I can explain this too. I tried to push the envelope and used a small toroid from a CFL but used enamelled wire from the same source. Because the wire was so fine, I figured it was possible to make lots of turns, say 50+. The result is a coil with ‘lots’ of inductance. I measured it as about 5 mili Henrys. This means in practice that the circuit oscillates at a lower frequency than it would with fewer turns. This gives a less bright LED, but probably increased efficiency. Less inductance gives a frequency that is higher and above the audible range. Mine is about 9 kHz and interestingly some can hear it and some can not. I tried muffling it with hot melt glue gun glue with some success. The frequency depends on the battery voltage too. It seems to increase in frequency as the voltage runs down. WPS