Air Con whistling

[3.7 slp]

Hi everyone

just wanted to see if anyone could help with this little prob. When i have the AC on i get a whistling noice coming into the car. I can only hear it with theAC on if i press the ECO button the sound stops ,which makes me think that it is related to the AC system.

Any tips on what it could be.I have had the AC system serviced and all seems fine with it ..just this whistling sound. it seems rev related as the car is coming to a stop you can hear it.

Thanks very much