Problem with using reverse

Change oil and filter first , NOW

Clear all Polecats from the rear of the car!

(Sorry, I couldn't resist...) ;)

Try lifting the car and turning the rear wheels forward and backward by hand. There's nothing in the back of the car that should be making any noise. Possibly a problem with the (drum?) brakes, or wheel bearing - may only show up under load, but try checking for noises with wheels raised. If you're in such a region, could be something as simple as snow packed in the wheel well rubbing against tire tread.