controlling air "bubbles" from Durso standpipes

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1) Are both 1.5" drains in the same overflow box?..or do you have 2 overflow boxes, each with a single 1.5" drain?

2) If you have 2 drains in 1 overflow, that's good..find out if 1 of those drains can handle the entire flow. Temporarily remove one durso to find out if a single drain can take the whole flow.

If one drain can handle the entire flow, then you are home free! There is a perfect solution for you that will silence your system completely. It's called the "silent siphon", or the "Herbie Method".

If a single drain can't quite keep up with the entire flow, you can still do it with a dramatic reduction in bubbles and sound, but it won't be completely silent.

If you need both drains running nearly full in order to keep up with the flow, then you are stuck with dursos and the other recommendations.