Howdy yall, I just got a used 2011 Subaru Legacy and it hasn't had any strange sounds, but after a huge rain storm a week ago it has been squealing lightly whenever I reverse. It doesn't happen when I put the car in reverse, only when I'm actively moving in reverse. If I press the brake to stop the sound goes away. It's at its loudest when I am just rolling backward, without my foot on the gas or brake. This sound never happens otherwise. The car runs smoothly and doesn't have any braking issues/other weird things so this feels like an anomaly.
Before I take it in to a shop, do you guys have any idea of what it might be? I appreciate any help or tips!

The Subaru Legacy has 4-wheel disk brakes with the rear wheels featuring a drum for the parking brakes. Have you checked the brake pads all around, the condition of the drum pads and the disk rotor/drum surfaces, the drum shoes are properly adjusted, made sure the pad shims haven't shifted out of alignment and the mounting shims are clean and straight, and there are no obstructions between the brake covers and rotors?