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When ever the throttle closes, the turbo is spinning so it has to dump the excess pressure that is building up, hence the noise when you let off the throttle, when you change gear your letting off the throttle so it sounds again as its dumping the excess pressure from the turbo spooling up.
Why letting off the accelerator twice?
 
When ever the throttle closes, the turbo is spinning so it has to dump the excess pressure that is building up, hence the noise when you let off the throttle, when you change gear your letting off the throttle so it sounds again as its dumping the excess pressure from the turbo spooling up.

Shaunboy, I think your right.

I've been doing some driving over the weekend and trying some different gear changes.

I've found that if i drop the clutch as i lift off i get one big release, but if i lift of then change, i get two small releases.

I am new to forced induction, so my driving style may need to adjust slightly.

Thanks for all you help though

Chris