About a year ago, my girlfriend asked me what one does with clicking batteries.

"Clicking batteries? What do you mean?"

"You know, when a battery just starts clicking. Like they do from time to time."

"I have never ever heard such a thing."

Two minutes later, she texted me a video. Sure enough:

I suddenly remembered a time years prior when I woke up in the middle of the night to a periodic clicking on my desk which had almost nothing on it... except a flashlight. In the morning the clicking had stopped, so I never could figure out the source of the sound until now.

The Electrical Engineering Stack Overflow has a quick summary of the issue; in nominal conditions, the chemical reaction in the battery should not produce any byproducts. But when the battery goes bad there can be a chemical reaction which results in hydrogen gas being produced, and the venting of the gas can produce clicking.

So to answer my girlfriend's question: clicking batteries have gone bad. We disposed of this one by taking it to our local hazardous waste disposal site.

Respect The Chemistry

Want to go really deep? The 2018 study Understanding the Dynamics of Primary Zn-MnO2 Alkaline Battery Gassing with Operando Visualization and Pressure Cells [1] by Faegh et al. describes the entire phenomenon in fantastic detail:

"The leading cause for safety vent rupture in alkaline batteries is the intrinsic instability of Zn in the highly alkaline reacting environment. Zn and aqueous KOH react in a parasitic process to generate hydrogen gas, which can rupture the seal and vent the hydrogen along with small amounts of electrolyte, and thus, damage consumer devices. Abusive conditions, particularly deep discharge, are known to accelerate this "gassine phenomena."

The paper also solves the mystery of exactly when hydrogen gas forms:

In this paper, we report that the formation of "black Zn" deposits with high surface area during deep discharge is responsible for cell gassing and leakage.

Most excellent results! I'm not an experimental chemist, but this paper was still pretty digestible to me, so give it a read if it sounds interesting.

[1] Faegh, Ehsan, Omasta, Travis, Hull, Matthew, Ferrin, Sean, Shrestha, Sujan, Lechman, Jeremy B., Bolintineanu, Dan Stefan, Zuraw, Michael, and Mustain, William E. [Understanding the Dynamics of Primary Zn-MnO2 Alkaline Battery Gassing with Operando Visualization and Pressure Cells]. United States: N. p., 2018. Web. doi:10.1149/2.0321811jes.