Well an engine knock (bearing noise) is just that, a noise simmiler to the sound of 2 big blocks of wood banging together. A tappet hoise similer to a hammer tapping steel.
How is you oil pressure and have you inspected your oil lately?

Oh a tip if the knock is heard (bearing noise) you can in some cases if it is a rod bearing not a main bearing remove the plug wires one at a time with the engine running if the noise changes you have probably found your culpret.
If not removal of the oil pan and a visual inspection of rod and main bearings can be made.
Unfortunitely cam bearings can't be inspected unless you remove the cam.

A tip on lifter noise. You can remove the valve cover and reconnect the plug wires then start the engine (yea some oil is gonna splash out so don't rev it up and dont waste time. Take a hammer use the wooden end (engine running) and push it against the push rod end of the rocker arm. if the noise changes you have found your problem.

Good luck

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"