Burping and glugging... my overflow

I've just installed my first overflow and sump. The overflow is glugging and burping. Best way I can describe the problem. That and a lot of splash.
The overflow is 200gph from marinedepot.com and the return is capable of 240 with the length of hose back to the tank.
I don't know get ut. It's as if the water is surging through the overflow.
I can not operate it like it is.
Any suggestions? I'm hopping it's a very basic thing I'm overlooking.
Thanks.