AC-USB Charging Device Makes Sounds when Device Fully Charged This might not be the right place to ask this question, as it falls under the purview of electronics, however that SE is not about consumer electronics. And this phenomena has only happened with Android devices, at least with the Droid 1 and Droid 3. So Android is where I stuck it. Sorry if that's wrong :(

This has happened with both the MicroUSB-to-USB (with AC adapter attachment) OEM wall charger and MicroUSB-to-AC-only wall charger.

Basically, when my phone is charging, the AC plug is completely quiet, but once my phone hits 100%, sporadically the plug portion of the charging apparatus will begin to make squealing noises. It stops once I unplug the phone and will begin again if I were to re-plug it in immediately.

Here's the thing, two other people I've asked cannot hear it. I promise you I am not going crazy and imagining things -- but I have been known to have very sensitive hearing -- hearing buzzing from old TVs and monitors and so forth.

My question, mostly out of curiosity, why is this happening and how can I get it to stop, as it is rather irritating.

This issue seems unique to my old Droid 1 and Droid 3. My laptop or similar high-power chargable device does not suffer from this issue.
See this very good answer on Electronics SE for details. Basically it's a side effect of the charger circuitry operation - it's by design, you can't change it. If that disturbs you - either find another charger, or bring this one further from you, don't try to silence it with an enclosure - that may lead to overheating.