Hearing a cavitation wouldn't cut it for me - I want to see it with my own eyes. Then I'd probably want to rename it. Sadly not all brake shops all impecably honest on these things. Rear brakes get so little wear it's unimaginable that you have damage on your rear rotors in 7k miles, especially with soft dealer pads. In any case you need a second inspection/opinion, starting with confirmation that the noise is coming from the rear.