So, if I'm seeing this right from your pic it looks like your problem stems from having to run your plumbing through a hole in the wall almost vertically.
Unfortunately, the quietest setup is the herbie but it requires a decent amount of vertical space. You could try siliconing an overflow box around the hole but I think the best you're going to get is a gurgling sound. I think the only way you're going to get around this is to locate the sump (or the hole to the sump) lower then the tank.
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