I'm sure you've already checked the valve clearance, but have you checked it while the engine is at operating temps?
Aircraft engines require valve adjustments to be made when the engine is hot, because the cylinder expands and changes clearances.
Probably not a big deal with the 230, seeing as how it has an overhead cam and no pushrods...but maybe something to check anyway?

Speaking of aircraft engines, my R1340 makes a very distinct knocking sound when the prop is turned over by hand, it almost sounds like there's a rod not connected to a piston.
Pretty loud too, when I shut it off and it slowly winds down from all that extra momentum from the 130lbs worth of prop, you can hear it in the cockpit...clunk clunk clunk...clunk.....clunk......clunk............clunk.................clunk.
There's absolutely nothing wrong with the engine, and I've had 6 different mechanics that said they all do that, regardless of tolerances.
Even had an old mechanic from WWII that worked on them when they came out of the factory say they always did that.
Only explanation they could give me was the rings slapping back and forth between the lands on the piston (there's 4 sets in there).
It still bugs the hell out of me though!
They say that with straight rings (as opposed to a tapered cross-section), that ring slap is bad enough to cause damage to the piston if you run the engine with less than 25" manifold pressure.
Anything higher than 25" MAP (it's supercharged) keeps positive pressure on the rings and seats them against the bottom of the ring lands to prevent any movement back and forth.

The reason I brought it up (mostly because I'm out of ideas)...maybe it's possible that you're getting some ring slap?
It could be well within service limits, and might not be anything wrong with it other than that it's making a knocking sound.
Like I said, it's a long shot because I can't think of anything else causing it.