Overflow return help

[Fragmented 99]

Using a Durso overflow and it works great and have an issue I thought someone may be able to help with.

I have a 120 with a 6" X 6" overflow with a return of aprox. 600GPH through a 1 1/2" drain.
The return is slightly under the water level in the sump but I am getting a large amount of air into the return water which is creating some noise. Any way to eliminate this?

Thanks
Tony