Squeeky/Whistle Noise - Help Please
 

Right i have a 1997 UK Turbo, when i accelerate to around 2800 revs there is a wierd squeeky/whistle noise that comes outta the exhast .... if i put my foot down to about 3500 revs this noise stops ...

But if i hold the revs between, 2800 and 3500 the noise continues ....

Any ideas ? getting baffled by this now

Any help would be greatly apprieciated .....

Thanks in advance ....