I've had a similar issue on my old CX bike (which I never did manage to fix, so take the below with a grain of salt).
I might suggest taking off the cassette, cleaning it if necessary, and reinstalling, making sure the lock ring is snug. Then I'd try fiddling with the barrel adjuster just a tiny bit to see if it either resolves the problem, or moves the problem to another cog. If it does move around (i.e. the problem jumps from cog to cog as you turn the barrel adjuster) then I'd definitely take it into the shop since changing the cable tension won't be able to resolve the issue (just move it around).
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