I've one of those small batteries used to charge mobile USB devices like cell phones when there's no accessible power outlet around. It's a Cellularline FREEPSLIM3000 model HY-L-3000.
Last time I tried to charge it, I heard an unusual sound, like a tenuous whistle. I've uploaded a recording here, it's a 12-second audio file file called "powerbank whistle.m4a". The sound can only be heard while charging the battery, not while discharging.
I tried to measure the peak frequency with my phone and if the app I used is accurate (no idea about that) there are two frequency peaks, one at 8k and one at 16k Hz.
Now, my question is obviously not about what should I do with my battery or whether there is a warranty on this specific product, but what electric component of the device could produce this sound?