Anyone with turbo knowledge?

I've got an mg zr with the 1.8 turbo k series. I fitted a manual boost controller as I don't have a boost solenoid fitted anymore. I fitted it yesterday and turned the boost to 0.5 bar which I believe is still less than stock boost. Anyway when the turbo hits full boost I get like a fluttering noise which sounds like the wastegate opening and closing quickly. Any idea what this could be. The needle on my boost gauge flickers up and down on 0.5 bar when it's happening.

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