I was getting same problem and this is what I figured out... 1st off I did a stockman mod as I couldn't fit a Durso in hang on overflow box, help BIGTIME.. now I figure that I have too much flow going into the over flow box so I do get a slight gurgle every once in a while so I'm going to go with a double overflow with double stockman hoping that will fix the problem all together... I did notice that when I slowed the flow (smaller u tube going to the stockman it was totally quiet so thats where I figure I need to 1" feed lines into the tank.
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Yamaha XJ650