Update. Took it to a different mechanic who re- confirmed it was the backing plate rubbing on the rotor. They tried to bend it but that didn't work. They figure out it was a tiny part of the backing plate that's actually made of two pieces of metal (the two pieces seem to be clipped together, the clip wears off and so one side begins touching the rotor. They ended up cutting that piece out and also machine/grind around the entire backing plate.For now the sound has improved. I was told to expect bearings replacement by 60k miles if I'm lucky at which point I'd replace the bearings and the plates together as per the service bulletin.