Loud single overflow

[Misha]

My blue marine coral 120 has a single overflow with a valve. I tried closing the valve and get a full siphon but this didn’t worked out as the water sometimes rise up and down. When open or partially open it makes an huge gurgling noise. Here are some pictures of the plumbing and overflow. Help and tips are really appreciated!

 

[Misha]

Btw the little pipe inside the overflow is the return line, I took the pipe that’s going into the display of