2000 Posts and climbing!!!
Join Date: Mar 2005
Re: Battery making noise when charging?
It may be out-gassing.... what was your charge current, and charge voltage?....
If the battery is full, and you attempt to overcharge it (ie - drive voltage beyond 14.5V), you begin to what's called "boil the battery"... except it's not actually boiling - it's actually electrolysis that breaks up the gel or liquid (both contain varying concentrations of water), into hydrogen and oxygen. The gasses build pressure, and eventually will start blowing by the pressure relief valve - which every battery has as a safety mechanism against overcharging.
If you do it for long enough, and enough times, the process will destroy the battery - especially if it's a gel cell. For liquid acid battery, if it's serviceable, you can add distilled water to it to bring it back to normal - but it's still not healthy for the battery if you do it all the time.