ahh I see. hard to say, but I think its coming from the micro bubbles in the filter socks popping and splashing salt water ...
is the glass piece that the bulkheads for drain are drilled in sitting flush with tank ? if you, place a piece of glass from there, to your skimmer, and see if that stops it. so you will have the right side of the sump open for air exchange.
very nice and clean looking tank by the way
