Over the last month or so i noticed an increase in turbo whistle, it was usually very vocal around 1800 rpm, I wasn't to worried so didn't investigate it any further as 9/10 its a non isisue.
But. as well as this is had two other symptoms.
A) When stamping the throttle I'd lose all power, with a big car jerk - taking foot off throttle and reapplying would see the power return.
B) I also had this increasing whooshing noise when accelerating, like the window was open a wee bit.
Turns out, I had a large split along one of my flexi pipes. It appears that the flexi pipe was rubbing off a metal surface over by the power steering fluid bottle, the mesh on the pipe wasn't really that worn but squeezing the pipe definitely gave the game away.
This was replaced (thanks Karl) and would you believe it, no turbo whistle, no whooshing noises and a shed load more power.
Result.