Come nearer thinking it is the wrist pin bushing loose. IF so, new rod except I am not sure you can still get that one.

I don't remember if C21 has the same bore as D400 if so, convert to newer D400 piston, the later 2 ring piston, and its' matching rod.

Walt Conner

The C21 is an F engine Walt, lots of parts for it. I just noticed it's noise because of how quietly the D runs.

"The C21 is an F engine"

I see, you only posted half the model number leading me to think you were talking about a "C" engine.

Walt Conner

A loose wristpin is the likely culprit.

Sorry Walt, it's a 1991 C21ZPN.

It could be piston slap if it has previously partially seized. When you disassemble the engine check the piston skirt to cylinder wall clearance and visually check it for areas that may have seized before.. Also check the wrist pin as mentioned above. Either could be the culprit. If the cylinder wall is deeply gouged it may not be worth fixing.

I had one of these given to me once with the same noise. I found that the muffler bolts were loose. I retorqued them and the engine purred like a happy kitten.

I once had a V engine that had a weird rattle to it. To light to be a rod knock, or so I thought. I kept listening to it until one day I was mowing and the engine suddenly went silent. The only sound was that of the blade coasting to a stop. I took it apart to survey the damage. Nothing was salvageable. Some of the needles from the rod bearing had somehow managed to remove themselves from the rod cap. Then, one needle went up through the transfer port and wedged between the piston and the block. It would appear that as the flywheel tried to continue the rotation of the crank, the rod cap broke. So I had a scored block and piston, broken rod, and a burned crank journal. I'd look into it before I used it much more.