Mine makes a "Flushing" sound only when the flow into the tank stops. Is this what you are talking about?
Make sure you add water to the overflow part in the external box before you start the small Rio or else you will suck nothing but air.
David
Ive got flow into the tank....just the overflow box's external part only has like 2 inches of water in it at the most at any given time....and its making a constant sucking noise.
Sounds like your return pump isn't strong enough.
Or, install a ball valve in line on your overflow drain to restrict the flow out a little. This way the water level will rise in the box and stop the noise.
