I had something similar to happen a while back and found that one of my pad retainer pins had backed out enough for the outer pad to drop down towards the hub and cause some low-speed scraping. I put it all back together and it backed out again. I finally had to replace the pins. Whatever is is, you should be able to see what's scraping/grinding on close inspection.

Just a thought: There is a stainless shim that goes behind the pads. Is is somehow possible that one of them missed the retainer pins upon reassembly and has dropped down to the hub?

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