No kinks or 90deg bends in the line? Try straightening it out a little if its flexpipe to the sump. Exit pipe not too far down? (Durso has said the bubbles trapping may be some sort of pressure thing.) I ended up drilling the top of the endcap and using tape to adjust the air entry. One day I'll put a valve there or something. Also lowering flow with a T after the return pump was an easy fix for me.

FWIW, Durso standpipe is just like this for me. It took a couple weeks of fiddling to dial in, and since it has been quiet for a year with no issues.