Mygiants, the reason egon and I suggested valving down a return line is to slow the water in that return since his overflows can outperform his pump. As you mentioned, it is pulling water so quickly air comes down with it. Ideally, if he could do a herby style, one pipe would be full siphon to match his return pump and the other overflow pipe would be above the water level to acts an emergency. that way you have silent pipes and you are covered in case of emergency. I believe that is actually the herby method and the 3 pipe method that egon mentioned is the bean-animal method. I am using the herby method and once i got my ball valve dialed in it works really well. If you valve it off too much, your emergency pipe kicks in, if you valve it off too little, its a regular overflow, but once you get it dialed in you dont have to deal with it very much. The bean-animal method is good if you have a lot of water moving for the amount/size of return lines you have.