Yep, you're having the same problem I did with a Mag on a closed loop. I never really could stop the chatter sufficiently so I sent it back and went with a good external magnetic driven pump. I found I could get a PanWorld for not much more than say a Mag18, and it seemed so much better for an external application. Also, I don't want to speak for anyone, but check with NaCl, I believe I read where he started with Mags and converted to PanWorlds for closed loops. Also, I have heard/read that Mags overrate their return flow rates, so don't necessarily be scared off when comparing an Iwaki or Panworld type pump that's rated only around 1000-1300gph to a Mag that's rated at 1800gph. In the actual application they may actually put out just as much or more. I'll let the physics/engineer types try to explain why- or the guys that have been doing this a long time. I was in your exact situation and someone on this board- I think it was Joshua- swore to me that a PanWorld rated at 1270gph would outperform an old Mag 18 I had- and he turned out to be right...

Btw, I'm thinking about selling the PanWorld I was just talking about to upgrade to a Tunze or Sequence pump. The PanWorld works great but I'm thinking I want more flow. Hope this doesn't sound like a sales job, honestly there are several people who I know would take it so I'm not trying to push it on you! But it is a great pump for a closed loop if you're interested let me know.