When the battery is charged, the electrolyte in the battery has a chemical reaction under the action of the current, and there is a heating phenomenon, accompanied by a sound like water bubbling and sizzling sound, which is a normal phenomenon not to worry about.
When the battery is charged, the electrolyte will be electrolyzed, it will bubble, and it will make a sound! The longer the charge, the more bubbles and the louder the sound! Maybe your battery is full of bubbles!