End play in the crank??..that can cause a knock at idle and seem to dissapear at higher rpms..
If it's a briggs engine with the counterbalance it's most likely crankshaft end play. (if all else is correct)
Quite common in both the OHV and the old L head briggs single cylinders with the counter balance system.
Very true: You need to check the crankshaft end play while the engine is fully assembled and take the crankshaft end play to the very minimum specs. In your re-build kit if you got the gasket set, the extra sump gaskets are different thickness so as to adjust crank end play. You can find the specs for end in the service manual.
I've had to actually make a metal shim washer and install it on either end of the crankshaft. Briggs has metal shim thrust washers. Easier to install the shim on the inside of the sump pan. Briggs usually suggests installing the shim on the flywheel end of the crank which results in having to pull the crankshaft completely out of the block. I'm not sure why Briggs installing the shim on the flywheel end of the crank, MAYBE because
the sump end of the counter balance has a raised groove at the sump end and the grooved mating surfaces of the counter balancer will have less play. (pay attention to your paper gasket thickness when doing the metal shim)
I have a steel plate that I bolt to bottom of the engine and rig a magnetic base dial indication so I can get the crank end play down to minimum specs.
You can usually see slight wear marks on the mating groove at the end of the counter balance. (sometimes you will still have a very slight noise at idle even after shimming but not a knock that sounds like a rod)
If you will post up a model, type, and complete engine info we can be more helpful. (with specs, etc.
That is a aggravating noise, even upsets gophers when on a mower.