help! boost leak or slipping belt?!?!

[Mark1]

Need some help please.

On fairly hard acceleration betwwen 2500 and 4000 rev there is a quite loud noise coming from behind the glove compartment area. At first it sounded like there was an boost leak but my mate reckons it sounds a bit like a belt slipping. its a low pitch rough, sounding whistling.

There seems to be no power loss and once you carry on past 4000 it stops.

The car is standard 225 s3 but with a remap done about 2 months ago.

Any advice appreciated. Cheers

Mark