If you're lucky it might not be a bad bearing.
I had a knock in a 235 that turned out to be nothing but a worn out woodruff key on the end of the crankshaft letting the crankshaft pulley cause a knock at idle.
If that's not it, start by raising the idle enough to where you hear the knock & one by one short out each spark plug one at a time. If you hear the knock go away or lessen significantly you'll have narrowed the problem down to that cylinder before removing any parts. If you find the source of the knock like this, it's caused by a bad connecting rod bearing or wrist pin bushing.
If you still haven't found the source of the knock this way it may be a main bearing.
Some folks use a stethescope or a wooden dowel with one end on the suspected site on the engine and the other end touching the outside of your ear. I hope this helps, let us know what you find,
Jacques
