New to the forum and asking for any ideas.

Ive had an insignia 2.0 cdti 160 for 4 weeks new although it's a year old and have had an unusual whining noise from the engine. Once the engine is warm, normal running temp I can here a whine or high speed spinning noise when sat in car when revs are taken above 1500. It doesnt matter whether the car is driven or stationary. If I put the aircon on the sound disappears but comes back after a short time when aircon turned off again. It's been in once and all they did was re gas aircon. It's not aircon issue as turning it on stops the noise. Any ideas