You can see the lead plates at the bottom of the hole, and the slot for the fill tube at the top of the hole. Now, sealed batteries, such as gel or AGM, certainly have the ability to make noise when charging.
Now, sealed batteries, such as gel or AGM, certainly have the ability to make noise when charging. However, a hissing sound (or anything indicating that pressure is squeezing out – like steam) is an indication that too much charge is being applied and irreversible damage is occurring.
With a flooded lead-acid battery the sound will usually become barely audible as battery reads 13.8 on the voltmeter (minimum voltage for charging). As the volts on the voltmeter increase, the bubbling sound will increase in intensity. Normal charging ranges can go up to 14.8 with a flooded battery.
When the battery can not absorb energy as fast as you are adding energy, the water in the electrolyte uses up that energy to make hydrogen and oxygen. There is no magic chemical in there that bubbles because it's happy or sad. It's just acid and water.
AGM batteries should not bubble during normal charging. That will eventually boil off the electrolyte and kill the battery. Bertus, sorry for the "hijacking". Didn't know it's a bad thing to do. I apologize.
Bubbling is merely the destruction of water. If the batteries are not sealed, replace the water with some really clean water like, "deionized" or at least, "distilled". If the battery is sealed, you can't fix the water level. They are sealed, VRLA AGM batts. They are sealed, VRLA AGM batts.
It probably has a AAA battery inside (so it has some voltage), or perhaps not even that. The rest is likely sand. The sound you''re hearing is probably the sand, or whatever garbage they stuck inside to make it heavy enough to feel like a real …