i did a little experimenting. this actually has nothing to do with the functionality of the airpods, so don't be alarmed. your airpods aren't on the verge of dying, it's a bit of a weird side effect. some people have pointed this out, but here's the exact explanation: chances are you heard this loud sound while having airpods in and laying down on them, and chances are also that you laid down on your right side. the pressure on the microphone and the eartip react, i'm not sure if this is intentional or a mechanical oversight, but this is the reason it happens. a few other users have mentioned this and i'll quote them below:
bennett166 says, "Don’t know if this had been answered yet but I’m pretty sure the noise happens when the AirPods are put under pressure and the noise is to help eject water"
gdcapo says, "It happened after a workout - right ear only. It had to be because of sweat. Same high pitched whistle. I went home pulled apart the Air Pods so they could dry out - for about 20 minutes - and that did the trick. This was the first time this has happened. Have owned them for about six months."
DFB77 says, "This happened to me when they got wet. In fact I put them thru the washing machine. So fairly severe. When they dried out it fixed itself up. I needed to use a hair dryer for a while but it worked."
a lot of people seem to find this issue while the airpods have been exposed to water / pressure in an enclosed space. if you'd like to test this you can take your right airpod and clutch it tightly in your hand, moving it slightly each time. eventually you'll hear a squeak. my final theory is that this is ANC reacting to a feedback loop, caused by water or pressure.