Thanks for the input. I figured that was the case with the higher-gear boost levels. The wastegate flutter would make sense too since changing the valve spring rate did nothing.
I was considering trying the Cobb XLE valve: anyone have this with a similar setup?
Also is the flutter even avoidable if it's supposed to be doing that, or would the flutter sound go away with a better seal? Seems like a silly way to regulate boost unless there's a technical reason for the oscillation like that?