The adapter brick is a DC/DC converter that makes sure the USB port sees 5 volts as required in the standard, while the older chargers the adapter is intended for use with can give out anything from 7 volts to nominally 5 volts, really like 6,3 volts - or something quite different, the old battery system wasn't that picky.

As an average high-efficiency stepdown DC/DC converter works in buck mode, i.e. switching the voltage on and off rapidly (tens of kHz or preferably more) - to step down just a bit, keep the voltage ON for much longer than OFF and you get a drop in output without significantly wasting power to resistive losses - and filtering the output, it's perfectly possible for the swithing process to produce audible noise. The noise is usually a sign of somewhat lazy design - it's quite easy to make sure the possible noise does not fall into the range picked up by human ears (and higher frequencies usually mean better efficiency too), but even with the best design there will always be some units in large lots that are always noisy due to e.g. component tolerances. Usually if a new unit is whistling, this is not a reason for greater concern, but if an old unit suddenly starts doing it, it is most likely dying and needs to be replaced quickly.

Changes in whistle pitch (in this case) are indicative of changes in charge current - the brick I'm using is almost perfectly silent when the device draws full current, but when the battery gets fuller it starts to whistle, gently at first and finally slowly chirping quite audibly when the charging is finished. Funny thing is that the chirping seems to be indicative of GSM/WCDMA and CPU power draw cycles - when e.g. scrolling the image gallery, the pitch is reduced considerably and when the scrolling stops, the pitch rises again (CPU working, drawing power vs almost no load and very low power).