the way Im reading it, the problem isnt with the plumbing inside the overflow box, it is the fact that the two drains are plumbed into one on the bottom side of things.....that is a major herbie no-no....two drains into one is creating back pressure which is why its causing noise and stability issues.........and just to correct what Denny said
....there is no syphon with a herbie....it is purely gravity that feeds it(like a bathtub or toilet)....
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260g mixed reef, 105g sump, water blaster 7000 return, Bubble King SM 300 skimmer, Aqua Controller Jr, 4 radions, 3 Tunze 6055s,1 tunze 6065, 2 Vortech MP40s, Vortech MP20, Tunze ATO, GHL SA2 doser, 2 TLF reactors (1 carbon, 1 rowa).
http://www.canreef.com/vbulletin/showthread.php?t=50034 . Tank Video here
http://www.vimeo.com/2304609 and here
http://www.vimeo.com/16591694