Yeah, the sound I'm talking about is from some other element of the electronics, not from the transformer (which is a fairly coarse buzz). You still get it when on third rail (and on the 769s). It sounds to me like a violin playing, it's just a really nice, relaxing sound. You only seem to get it on 319s, not on e.g. 321s.
On this YouTube video:
...you hear the low transformer buzz first (not present on diesel or third rail) but then the other sound I'm talking about starts shortly after.
You can also hear it on this video which is taken in the transformer car of a 769: