Acura Tsx whines when you are backing up

Hello everyone,

I have a question regarding my wife?s 2005 Acura Tsx automatic transmission, the car whines loudly when you are backing up, it does not happen all the time but it is a loud noise and it stops if you hit the brakes, I went to the dealer and they said that the noise is from the rear brakes although they have lots of life left and I need to change them cost $120, not convinced I did not do it.

I hade the front brakes done last year under warranty as the car has now 35000 miles.

please let me know if you know what it might be, the noise is not the same brake noise I hade before when I had the front brakes done, it is louder than a squeal and it does appear to be coming from the back of the car.

I appreciate you help, Thank you