I vote with Klaus. I have a 2003 V70 that does the same thing on certain days and certain weather conditions etc. During high vacuum situations, like decelerating with my foot off of the throttle, I get a faint high pitch whistle from the brake booster. The SLIGHTEST touch of the brake pedal (maybe even pushing side-to-side with my foot) cancels the whistling. I think mine must be leaking around the brake pedal shaft vs the master cylinder side. It rarely occurs, never affects braking, and doesn't leak enough vacuum to trigger a code/check enging light. I just relocated to a dryer climate so it will be interesting to see what happens. Eventually it will become annoying enough to send me to the pick-n-pull to get another booster - probably one with the same whistle. Volvo used to sell just the seals alone for those things but those days are probably long gone.

BTW - some people have reported that a clogged PCV system will cause a whistling noise, but your car is low mileage. So it must be more age-related than mileage-related.