what is that sound? How can I tell if its a bad battery or something more serious?

My battery went dead on my truck.
When I put it on a chrging unit the battery started to make a "popping" sound almost like the sound of an oil can so I disconnected the charger immediately. What would cause that sound???
I am able to boost the battery from my other car but wont hold a charge. How can I tell if its just a bad battery and not something more serious???