Like most batteries, Alkaline batteries produce electricity through chemical reaction. This produces byproducts like liquid and gas. The former can be seen when a battery goes bad and leaks out. The latter can be heard. It's normal for hydrogen gas to be produced. The batteries are not sealed, to allow this gas to vent. But misuse of the battery, or if it goes bad, excessive amounts of hydrogen gas is created. This, along with liquid corrosion byproducts, can cause gas bubbles, which you can hear.
Figure 2 is a cutaway drawing of an alkaline cell which uses a seal that consists of a vented plastic grommet. An alkaline battery requires a vent because there are circumstances in which it must release gas byproduct. Our lithium cells can be hermetically sealed because gases that are produced inside the cell are quickly dissolved in the electrolyte, and because the speed of the chemical reaction is limited by the inability of the cell to produce high current.
NiMH Vent:
Update:
I personally experienced this lately, when I modified a solar light circuit to use as a Joule Thief. I connected two AA Alkaline drained to 1.1V each to a YX8018 solar charger and led driving circuit to bypass the 0.6~0.8V UVLO feature and drain those suckers dry. One cell drained down to 0.3V @ 8mA by the next day and started leaking and clicking. Same half second frequency. Word to the wise, don't bypass UVLO circuits. I don't expect it to explode, but cleaning battery gunk isn't fun.
