Possibly some images will help, these are a bit different from any overflows I have seen.
This is the overflow as seen from the front of the tank:
This is a view from further back:
And this is the view from the top:
It is odd, it will go for a long period of time making the sucking noise and then be silent for a long period of time. Last night it was making the gurgling for a few hours and quite frankly had me wanting to buy an aquarium with a better overflow as I have a feeling this would be considered bottom of the barrel quality wise.
Any insight into this overflow system is appreciated.