Boiling sound from battery?

[jrringha at adobe.com (John Ringham)]

Yesterday morning, I hooked up the battery charger to my car since the
battery was dead. I let it charge at 2 amps all day. The charger has a green
light that comes on when the battery is fully charged. It wasn't on yet, so
I flip the switch from 2 amps to 10 amps. The green light comes on and the
battery starts making this boiling sound really loud, sounded like it was
going to blow up. I quickly turned off the charger, and felt the battery to
see if it was hot. Nope. Any idea what that was? Freaked me out.