What turbo are you using and at what boost? Is the flutter sound on or off boost? Flutter is fine when you shut the throttle as long as all your connections are tight and your turbo isnt already shot. Despite popular belief surge when the turbo isnt loaded is okay, when the turbo is loaded and surges it then causes major thrust issues. I personally love my turbo flutter off throttle due to the lack of a blow off valve. Best sound i think. If youre surging during boost then you got some flow issues. Mismatched turbo could cause this as well as smaller i.d intake piping with large turbo. If youre trying to shove air at a rate higher than which it can be consumed then its likely going to surge so id start looking at possible restrictions. Verify throttle position, see if the intake is caked full, peak at your valves through your exhaust ports, stuff like that. No idea how not stock your ride is but hopefully helps you narrow down what youre looking for.