Charging a 24v (2 -12v sla's) system with a "LiFPo4" charger... Problematic?

I am not totally sure here, but I dont think you are supposed to use a charger thats not made for the kind of batteries you are using. Gurgling sound is NOT normal in my experiences.

I would be careful and not use the LiFePo4 until you are certain it can be done safely.

As best I understand it, many lifepo4 chargers have a different charging profile from sla. The lifepo4, has more current near the end of the charge cycle and will boil away an sla and ruin it. The reverse, charging lifepo4 with sla, can be done, but it will take longer, as the sla charger slows way down at the end of the charge cycle. Stop using the lifepo4 charger for the sla's. It might be ok, to use it for a partial opportunity charge, like while you have a cup of coffee, but don't run it to the end of the cycle ever.

What dogman said is probably right on - another thing is that a LiFePO4 that's close to 24v is gonna actually be 25.6 volt nominal and the charging voltage is gonna be much higher than the max charging voltage of the SLA pack.

Hence, using a LiFePO4 charger to charge SLA is going to overcharge the SLA.

Thus the likely reason for the 'gurgling' is that the batteries are overcharging and whatever liquid is inside the batteries is boiling. Do that enough times and the liquid will boil off and the battery will be useless.