Although I haven't seen the innards of a BAP, as an electronics geek I'll say that the BAP probably uses what in electronics design is termed a "boost converter" which is a circuit that allows one to generate a voltage higher than the input voltage (that is, it generates 17V output for a 12V input.)
Boost converters have magnetics (inductors) and high frequency switches (MOSFETs) in them as part of their circuit. The noise you're hearing is these components vibrating as a result of this switching action and current flowing. Although it's technically normal, it's a sign that the circuit components may not be deeply potted or are mechanically "loose", such as an inductor whose coils are not tightly wound to the core or a FET that's not tightly fastened to a heatsink.
You can hear a similar effect when you walk by a large power transformer and hear it humming. In that case, the core of the transformer is moving in response to the magnetic field created by the current flowing in the windings, switching polarity at 60Hz and causing the whole structure to vibrate.
The high pitched noise can result when the switcher enters various modes of operation as the duty cycle of the FET is changed in response to voltage output and current demands.
I hear a similar high-pitched whine from my BAP too.