Not a clunk, it's a clank. This post is clank-bait.
I tried clickbait once. I started a thread called Live Nude Girls to capture the attention of all the people who already tuned out of my other threads. I got yelled at. LOL
I think I figured it out. There seems to be something I was previously unaware of. Gear Backlash.
I think, If I understand correctly, that if the gears are too small or too far apart, then you get something like what I have going on.
If the gears are too big or too close together you get squeaky, grinding noises .
"Whenever the roller follower on a cam lobe passes the maximum
lift point, the forces on cam drive gear teeth change direction. If
more than .002 inches (.05mm) backlash is present between the
cam and pinion gear, this directional change of force will result
in an audible "click" as the backlash moves from the back side of
each gear tooth to the front side.
While some positive backlash is necessary to prevent localized
gear tooth overloading, excess backlash (and "clicking") may
sound annoying but won't hurt anything."
http://www.andrewsproducts.com/sites/www.andrewsproducts.com/files/catalog_pdfs/page_19.pdf
