I had the same issue with my brand new Tamron 28-75 lens. It feels exactly like you describe it, like subdivided in many little steps or a sprocket in use for the focus ring.
I sent it to Tamron Germany for service and didn't hear from them 6 weeks. After a claim, I was told "we are waiting for spare parts from Japan", but two days later I got the lens back - obviously untouched and without a remedy for the issue.
But manual focussing works in spite of the strange noise and the feel, and AF is spot on, so I decided to leave it alone.
BTW, a fine piece of optics.
So, don't worry unless you notice to have missed focus in MF.
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