Subaru Outback Wagon - whining sound in reverse

My 2004 Outback Wagon started making a loud whining sound the other day - but only in reverse. I’ve been told that it’s the brakes. Anyone know???

I would be more likely to suspect a dry differential (most likely the rear diff, but possibly the front diff), but only a qualified mechanic who can actually examine the car can tell for sure.

When was the last time that the differentials were serviced? Subaru specifies this maintenance for every 3 yrs/30k miles.

This car needs to be checked over carefully by a competent mechanic, before something actually does break.

Manual transmission? Whine from the front, rear, or console area?