What should I be looking for to correct a bad knock on a rear fixed wheel?

I've got a rear fixed wheel with a bad knock. Cone adjustment has reduced the knock, but not eliminated it. Bent axle? Other ideas? Thanks!

A bent axel will give the sort result where bearing preload adjusted in one position of the axle's rotation will be change when the axle is rotated to another postition. Try pulling the axle assembly out and removing cones and locknuts. Then roll the axle on a flat surface to spot wobble. If you spot the wobble then replace the axle. Let us know what you find.