Sorry AKliving, I have not had a chance to circle back since the last time - it looks like there is quite a bit of air being taken into the overflow down pipe. You should be able to resolve this by slowing closing the gate valve (it should show OPEN/CLOSE arrows on the red circular dial).
If you slowly close the valve it will increase the level in the main aquarium so that the red tube in the display overflow with the clear plastic box around it is no longer able to suck in air. You can start by making larger adjustments to get 'within range' and then you will want to make smaller/fine adjustments (while waiting at least a minute or two between changes) until it is quiet, but also still flowing through the sump quick enough that your return pump does not start sucking air from the other end.
Over time you will begin to get a feel for where 'it should be' and it will become less cumbersome and faster to set. It will require re-adjustment every so often - likely when doing water changes or other similar things.-
The last thing I will drop it a link to this video (the video is the old valve but similar idea)
If you are still struggling try adding more water volume to the tank - as long as the sump does not overflow when the return pump is turned off you can keep adding water (obv you dont want it right at the rim, so this is 'within reason')
Hopefully something there helps
