Soft whine from rear of car, 2002 XC70

[FritzMissel]

I have noticed a soft whining noise coming from, I think, the rear of the car. I brought the car back to the dealer (recently purchased used). They let some air out of the tires and the sound went away...now it's back and I have not changed the air pressure. Only happens between 45 and 55 MPH for some reason. Does not change when I put the car in neutral or shift. Sometimes changes on acceleration. Can't make sense of this. Thanks for your help in advance! I can ignore...just want to be sure it's not a problem while this thing is still under extended guarantee.