whistle noise????

[scoobyfuzzy]

ok guys, over the last few days ive noticed ma motor make a whistling sound when accelarating up intil i change gear, starts when im at about 3500-4000 revs, any ideas wot it could be? i know with the decat u get a whistle, but ive put the standard exhaust back on, could it mayb b a turbo problem or dump valve??  still boosts up2 10-11 psi  which its always done. any ideas guys, oh its a my93 import aswell

 

cheers

 

fuzzy