Here's my guess. And it's just a guess since i don't know anything about the electronic design involved.
You're perhaps young or still have some good hi-freq hearing left from not being exposed to loud noises (gun fire in the military, loud music, jet engines, racing cars, mowing lawns, etc. -- i've nearly always worn hearing protection, myself)
2D = 3V which is too low to forward bias the LED. so, my guess is that some type of switching supply/electronics (a boost converter of sorts and perhaps a regulator too - like i said, i don't really know since i've never seen a schematic diagram of the electronics involved) is involved (or in some cases PWM, but probably not in the case of a single level of light output as in the MagLED - some PWM is too high to hear as is some switching supplies/regulators).
these control/produce a proper voltage level by switching at very hi-frequencies. When i put some PWM and other flashlights with switching supplies in them on a JakStrap near my right ear/temple area (i'm an old geezer, so it needs to be no more than a few inches away, generally), i can also hear an annoying hi-freq noise. Very annoying. Eventually, i subconsciously tune it out and don't notice it any longer. For example, a Proton flashlight, currently near my right ear as i type, when on max, i think i don't hear anything - IF(??? not really sure i can hear it - you know how very hi-freq is - more like a sensation or pain than a sound - at least that's how my brain interprets it), well anyways, where was i, oh..., IF i do hear it, it's really, really high freq (17 kilohertz or higher would be my guess - yeah, take care of your ears and, even though it's downhill to a 100 for me, as far as age goes, you'll be able to still hear such higher frequencies - though not as clearly/loudly/distant). as i dim the output of the Proton, the freq gets lower and i definitely can hear that annoying buzz/noise/hi-freq waveform.
the most annoying ones of all are the Nite-Ize IQ switch for the AA MiniMagLites. talk about noise pollution!!
i've read in some other Posts of a similar nature/complaint with other lights. my guess is that in these cases it's the electronics controlling the voltage.
don't worry, as you age, you won't hear it as well anymore!!!
EDIT:
turned Proton off for a while. then, turned it back on. at max, can definitely hear a buzz, but i'm not so sure about it being as high as 17KHz - probably a somewhat lower freq. so much, for my initial intuition!
