Noisy drain to sump

[Leon71]

I've researched lots here and other forums to find out how to stop the noisy drain in my new setup. Just some history.

I have a Marineland 120gal with a corner overflow. The plumbing came with the tank and is a type of durso system. I have 1" bulkead through the bottom of the tank, followed by 1.5" standpipe and two 90° elbows at the top. The one elbow has two 3/8"-1/2" holes (one on either side) just above the standpipe, and a vent tube at the top. I can't get it to stop sounding like a toilet drain. So I replaced the elbow with a tee and cap. This seems to be common practice, so I thought I would give it a try. The top is now pretty silent. On the bottom I have 1" PVC straight down from the bulkead to the sump. With a union inline.

Now the problem. On both setups, I have extreme amounts of air coming down into the sump. I'm talking 2-3" high "boiling" in the sump. I've tried a shorter downtube (just at the water surface) and a longer downtube (3" off the bottom of sump, in 14" of water). There doesn't seem to be any change. Any help would be greatly appreciated. Am I overlooking something?

Thanks, Leon