Minn Kota MK-330 question

I received my MK-330 charger but don't have my new boat yet so I figured I would just hook up the charger to my old boat batteries and 1 from my tractor to try it out. 2 banks said charged which I expected and 1 bank went on charge, after about a half hour the charger was buzzing pretty loudly, is this normal? It was only charging on 1 bank at 10 amps and it was way louder than I expected, if all 3 banks were running and each of them sounded like that.....well lets just say not what I expected. If this is normal that's ok, I just want to make sure. I think I will switch the batteries around an verify if each bank sounds like that. I hope it's normal as return shipping charges will eat up the rebate they have going. Thanks for the help.

mine is the same loadness weather it's charging one bank or all three, so it's prob normal

Yeah I switched them all around and they each do it so I am guessing it's normal as well. The temperature sensors really work too, I was monitoring it and it was up to about 14.6v then I grabbed the sensor in my hand and the voltage dropped and it shortly went into maintenance mode......sweet! Pick up the new baby bug 4 days. Thanks