I don't use airline tubing either and it sounds kind of like a faint whistle.
I'm not sure about this, but from the pictures, it looks like your standpipe is half the height of your overflow box? I'm not sure this has the height to draw enough air being that far down, but it should really be about the height of the overflow. Also, the standpipe should be a bit larger than the bulkhead (0.5" at least) if you wanted it to handle more flow.
Another thing you might want to try is put some eggcrate and plastic mesh at the end of the drainage line in your sump. This should break up the size of bubbles and help redirect the water away from the pipe. I've found that this stopped the burping in my plumbing and the bubbles created are diffused by prefilter foam. This makes a faint "fizz" sound but I can't hear it when the stand is closed up.
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