Seen this before!
Long version:
I would bet on one(or more!) of the cells having run dry, either
through lack of maintenance or age. So, the dried out cell/s has/have
now gone short circuit resulting in the charging voltage now being
shared by 5(or less!) cells effectively instead of 6. So the charger
is attempting to charge a now 10(or lower)volt battery to level
appropriate to a 12 volt one, resulting in overcharging the remaining
working cells. The sizzling will be the sound of the dry cell/s, and
the remaining working cells will be gassing profusely, and getting hot
to boot.
Short version:
Yes! It's knackered!
Neil.
(Reply via group please)