the fork is rubbing on the release bearing, this is what I've seen when changed the clutch recently. the mechanic greased the contact area with copper based grease and guess what: problem solved
and guess again: it came back 2 days later.
what bothers me is how on earth a friend of mine's 172 doesn't suffer from this and he never replaced his clutch, and has some extra 6k mileage on. I'm not doing track days and 90% driving normally