My Nokia cell phone is charged with a very lightweight compact charger similar to this AC-15 (can't find the one I actually use).
While the battery is close to empty the charger works silently - just heats up a little. Then, as the battery charges it starts to produce silent yet audible chirps and the more the battery charges the more time these chirps take and less time the charger is silent - something like one chirp with duration of one second, then one second of silence. Finally as charging stops and the phone says "charging complete" the charger starts producing a continuous tone of the same frequency as those chirps. When I unplug the charger the sound fades within about two seconds.
What component of the charger circuitry produces those chirps and why and also why would chirping pattern change as battery charges?