Thanks for the suggestions!
nitpicker330; It is worth bearing in mind for another occasion, but there was one catch this time around. The APU was off.
Uplinker; I managed to talk to someone in maintenance more in the know of the A319's electrics today, and he informed me that the power that arrives in the horizontal stabilizer for the logo light is, indeed, 115 AC (this checks out in the FCOM), but it is stepped down locally (in each horizontal stabilizer) with a small transformer to 28 V for the light itself. Whether it is 28 VAC or 28 VDC he was not sure. I asked him that particular question as I know from years ago that "bulbs" are indifferent to AC or DC and will work equally well on either. I asked him to please do me a favor and check it with a multimeter the next time he was doing anything around that area to determine the type of current, or look a bit closer in his texts for the answer, as I am curious now. Anyway, the noise was coming from a faulty transformer, he said.
I would not have imagined it could make such a racket; during the walk around I initially thought it was a constant siren from some hangars in the distance, until I was under the horizontal stabilizer and located the sound, after which it seemed to be the exact same sound as the yellow system pump.
Totally new to me...