Clarity Plus overflow question

Hello guys,



I got a used 100g Clarity Plus hex with built in overflow box and I am trying to figure out why the overflow drain is so loud. The tank currently has the bulkhead at the bottom of the tank and I basically have to restrict 90% of the flow from my pump to quiet the constant gurgling. If I restrict the drain valve slightly, it fills the overflow box and the noise goes away but it slowly fills to the top, never reaching a balance.

Is the over flow missing something? It seems that other overflow boxes from other tanks have one pipe that goes to the top of the overflow and mine has nothing at all.

Thanks in advance.