Octavia VRs mk2 Turbo whistle

Hi guys few weeks ago my turbo started to make a whistle when it spins up on my 2006 octy vrs , not sure if power seems off, possibly.

My first thought was a split diverter valve as its never been changed and car has done 52k now.

Anyway im not very tech savvy so i took it to my usual garage to look at and suggested about a split Diverter valve.

Got it back and they cant find anything wrong with it, they done a smoke pressure test and found no leaks.

If its the Diverter valve would that come across in a pressure test?

any other suggestions as the mechanic didnt give any?

Cheers in advance

Pete