I've driven cars with bad ball joints that only give an indication when backing up with the wheel turned. It's a difficult sensation to describe except to say that it feels like the front "shifts" to the side. That would change the alignment of the front wheels while backing, but it doesn't sound like your problem.
Sloppy bearings could let your brake disks shift slightly in relation to the calipers/pads and cause some noise. Especially if the rotors are near the end of their life.
You didn't mention if you get the noise with the wheels turned while the car moves forward.
I might suggest that you might have an issue with the control arm bushings, but if a shop checked out the car and did a good job of it, those should be obvious.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)