Charging AGM Batteries

Not sure what happened but here's the go.
I've got 3 AGM batteries and I keep them charged at least once a month. I know 2 of them are reaching end of life.
So I put one of them on the charger and it took a few hours but I started hearing a popping sound coming from the battery. Unplugged it and tried the other. Same thing happened.
It can't be the charger, I"ve got 2 and the same thing happened with the second battery.
I'm not an expert with batteries, anyone seen this before ?
Cheers