adalton Thanks! Hopefully I've gotten to the bottom of this problem.

You fixed it by replacing the gasket? It looks like the gasket on mine is still in place and the hole for the stud/bolt is clear. Do you see any reason I couldn't just fit a new bolt and see if the noise goes away?

 

you could. These don't seat right ever with just 2 bolts. Though replacing the gasket and the stud isn't too hard usually, just soak it all with a penetrating oil and even heat cycle it a few times in-between applying the oil and should be pretty smooth.

 

You can just install a new bolt and tighten it down. Should get rid of the noise. That was the case with me.