Why is my DC motor making a rattling scraping sound?
Its almost always bearings. chances are if its scraping already, they'll be right full of tiny metal filings in the grease. If you have the luxury of being able to dissassemble it somewhat, check them all with an IR thermometer or similar, the hot one is the one you want to fix, but perhaps you should give them all a little grease for longevity's sake.

then pull the bearing (there are tools, but sometimes you just have to get creative), and just find the same bearing again online... just like electronics, everything's marked and described on the part... or there's a datasheet.

I'll entertain the possibility of the brushes being worn out too, but that tends to result in near total failure rather than rattling and scraping