[banditpowdercoat]

You need to make it a Durso style drain. A open bulkhead like that(the green one) will allways fill up till it makes a siphon, then gurgle. Where the 90 is, put a T, with the top of the T capped and drill a little hole(1/32-1/16 Or best, a Adjustable shutoff) to regulate the air coming in so that is cancels the siphon. Doesnt seem like much, how your draining from the fuge to the sump, but its in essence exactly the same as tank to sump, and can make just as much noise

PS, Just drilling a hole in the 90 will work too, If you add a tube to it to make sure it comes above the waterline. Just drilling a hole, chances are water will flow out the hole, instead of air entering