controlling air "bubbles" from Durso standpipes

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It sounds like you have the air adjusted at the right level if it's stays steady halfway up the elbow. If you restrict the air it's going to rise and fall and sound like a toilet flushing.
How far is the water falling in the overflow? Maybe you can raise the level with a longer standtube might that might help a little.
I use two 7" filterbags on my two returns from a 2" Durso and they totally eliminate any bubbles in the sump.