you may need to get someone with experience in these units (or an electric forklift tech with a schematic) to check your battery hook up. this is not so much a 24 volt truck as a 2 section 12 volt truck that can use both 12 volt sections together as 24 volts for a faster travel speed.
my bet about the 'popping sound' is that you are blowing the circuit breaker inside the charger, and if you let it go, it will blow/burn out the circuit breaker.

You have to have the disconnect lever up and connected or else both sets of batteries will not be connected to the charger. What happens if the lever is up and THEN plug in the charger? How high does the needle go up? Does the needle stay there? Where is the pop coming from? Does the charger pop and then the needle drop down?