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..could it be the spacer spring thing? Clearance of pads to disc looks fine, no rubbing but there's a light "zhing" noise when under way. Closer examination looks like the folded spring that holds the pads is moving about. (Shimano Deore M6100 if that helps). Of course, didn't have any pliers out on the trail to pull out the split pin!
I've only ever had issues with the retraining spring when the pads are very low and then it was more a ticking noise.
Is your rotor slightly out of true?
...or a loose calliper bolt?
didn’t have any pliers out on the trail to pull out the split pin!
You can use a small Allen key thru the loop to pull/lever it out. Or a zip tie. Or a shoelace. Or nearly anything to get a bit of grip on it. Or take the wheel out and tap it thru from the back with something.