Battery crackling sound

I can hear sounds coming out of at least one cell of my 2 yr. old Repro R-59 Delco battery even after it gas been disconnected overnight. Sometimes, I find leaked battery acid on the top of the battery, I just thought it was odd that it would continue to make this sound even after being disconnected for 12 hours or more. This also leads me to another question. If it is an overcharging situation, how do I hook up a meter to check it?
Thanks,
Rich