If your engine is knocking, this is an old fashioned, but effective way to determine the location of its origin. Don't laugh, I've tried it, and it actually works very well.
What you can do is take a wooden broom handle and stick it on the engine in different places like a stethoscope. While doing this, hold the other end of the broom handle up to your ear and cup your hand around it to your ear, and listen to the wood. Try this is different places and you will be able to pinpoint exactly what part of the engine is making the loudest noise.
Thanks to by friend Rex Floyd of Floyd's Garage - British Bike Repair and Restoration for the tip.