cooked

help please with my 99 td5.when it starts in the morning and it sound like their is a blow on the exhaust.as you pull of and rev the engine their comes a noise it sound like a knacked bearing starting at 1900 to 2000 rpm and gets louder the higher you rev the motor.as soon as you change gear of the revs drop to below 1900 the noise goes.the noise does not do it ever time or in the same gear or revs. if you are gently with the throttle until get to 2500rpm then hit the motor hard the noise does not come.their is no lack in torque but their is a small loss in power.i went to brum today and i must of been half way their before it started squealing.as i went pasted a merc at 85 mph all was well with no noise.as i accerated of a traffic island the noise was their in 2nd,3rd and 4th.when i got back i could not hear the exhaust blow but reved the motor to 2500+rpm the noise seamed to becoming from the turbo area of the engine.has anyone got an idea of what it is.the engine in not smoking