Found a neat site that walks you through disassembling your electric toothbrush and replacing the battery. Usually requires some soldering, but... very cool. (They're funded by selling replacement batteries!)

https://toothbrushbattery.com/

ToothbrushBattery.com - Replacement Toothbrush Batteries

Click here to buy a replacement battery for your Braun Oral-B or Sonicare electric toothbrush. All sizes available. Worldwide shipping!

ToothbrushBattery.com

While disassembling my toothbrush, I came across an oddity. What are these little gray cylinders?

They're held loosely captive in a pocket of the plastic, not touching any other component. They're lightweight, rigid, non-magnetic, and maybe composed of something granular.

What the heck?

[EDIT: See rest of thread for possible answer!]

@varx best guess is to rattle so you know for sure it's on.

they don't look dense enough to be ballast but that'd be my second guess.
@greycat This thing is *so loud* (with or without them) there can never be any doubt. :-D