huskavarna

It is very possible that the connecting rod bolts have worked loose and need to be addressed. There is a small chance that the upper crankshaft bearing and or journal have worn enough to allow the flywheel to contact the magneto, or other external protrusion close to the flywheel, even a shroud bolt that is too long may contribute to this.

It might also be bent crank and or pulley if the intereference is noted externally.

At this point, do not run it any more until the problem is located and fixed. At a minimum, you do likely need to pull the engine if an external source for the interference is not found.

If the pulley looks to be pulling apart, that too can be a source of knocking noise, and that would be at the speed of the shaft it is attached to. There may even be a visible bright spot on a high portion where it is hitting if it is actually off center and off true.