Well, I think I figured it out. The unit can take several hours of running to evacuate all of the air. I let it run a while longer and the snapping ASO is getting quieter and less forceful. It's almost gone now, and I can tell it's just air by the more familiar sound as it bleeds out.
It never bled air so loudly before without the post-filter, so the ASO built into the post-filter has a very positive shut-off. If your Merlin is sensitive to low water pressure (running to drain after closing) this inline post-filter may fix it for you.
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