Alloy freehub shell and steel cassette cogs cutting into the shell during pedaling. It's very common, generally a light rub with a fine toothed file will remove burrs to allow easy reinstall of Cassette.
On my dads MTB some brilliant mechanic installed a SRAM 9 speed cassette with one cog missing, meaning you could not tighten the cassette at all. He used the bike like this for years. It rattled like hell...
His freehub looked similar to this. I installed a new cassette, didn't change the freehub. No problems at all.
As the others have said, that is from the individual cogs on a loose cassette, most likely judging from the lack of it below the 4th cog, an SLX. As said, a fine file will take the edges right off and if you want to mitigate this in the future get a higher level cassette which has more cogs pinned to alu carriers like an XT and/or see if you can get a steel freehub for your hub.
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