2006 Subaru Forester High pitch noise

Hi.

I just purchased 2006 Subaru Forester L.L. Bean edition. I only have

I started hear "airy high pitch noise" from rear under of the car. (it's almost right below the cargo area). it's never loud and happens after I drive it for 5 minutes. I can hear it even when it's idling so I can assume it's not caused by the roof rack.

I read somewhere that if the rear differential heats up, it might make that kind of noise. does anyone have any idea?

Thanks.