I have Nexus 5 and I recently bought the European version of the PowerCube, which combines three power outlets with two USB chargers. I noticed that as soon as my smartphone's battery is full, it starts producing a high-pitched whistling noise.
It stops when I unplug the phone or when its charge drops below 100%. Neither of my parents can hear it because it's outside their hearing range. Should I be worried?
This is indicative of poorly designed/ defective piece and best replaced ASAP.
From bwDraco's answer on Why do some AC adapters and power supplies generate a whining noise, and what can I do about it? (emphasis added; missing text denoted by "....")
Most modern AC adapters are switched-mode power supplies. The internal switching frequency of an SMPS is typically low when unloaded and increases with load up to a certain point depending on the design. The no-load frequency is often low enough to be within the human hearing range....Under load, a properly functioning SMPS should operate at a frequency well above the human hearing range, typically 50 KHz or higher (although some older designs operate at 33 kHz). However, the same noise can occur under load with a poorly designed or defective power supply as the coils may vibrate under electrical stress at a subharmonic frequency....As such, a defective device can generate audible coil whine during operation
From iHelix150's post on Why does my phone charger make a horrible high-pitched noise when it's plugged in? And can I stop it?
Most switching power supplies use a switching frequency above 20-30KHz so the human ear can never hear it. However for one that is in the 10-20KHz range, the switching is sometimes translated into audio frequency noise ....The noise changes based on how much power is being consumed by your phone....Unfortunately the way to stop it is to just buy a new cell phone charger, ideally one that's not a cheap piece .... If your phone charger is not a cheap piece .... (note that whether or not it's a cheap piece .... has nothing to do with how much you paid for it) its possible its capacitors were damaged by spikes, surges or other bad quality power problems, in which case you should invest in a good quality surge suppressor with noise filtering.
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