'05 Pacifica front wheel drive, noise at the rear when start backing up

What is your question?

every time I put the car in reverse (only reverse) and start backing I hear a whaling/welping noise at the rear end. I checked the brakes and they're ok. It shifts smoothly in/out reverse, I hear the noise only when start backing up...you can easily hear it from inside/outside the vehicle. Any suggestions on what might be causing the noise?