Consider getting a mechanics stethoscope. $20 for a pro quality one. With the 'scope you can tell exactly which cylinder, which rod end, even which valve is acting up. A very handy tool everyone should have.
The sound of the knock should give you some clue. A low frequency "thump" at idle that tends to go away with revs would be a rod knock. A higher frequency "rattle" that increases with engine revs is probably a valve train issue.
Did the engine power or performance change? The engine will feel normal with a bottom end problem. A top end problem will tend to limit your valve travel or not open the valve at all and cause a cylinder to loose power.
Which head do you have? If you have a complete turbo engine you have either a P90 or a P90A. The P90 head has a standard valve train with the mechanical valve lash adjusters. The P90A head, the latest turbo head, has hydraulic valve lash adjusters. Most of the aftermarket shop manuals (Chiltons, Haynes) do not address the P90A head. If you hunt, you may get lucky.
"If you ain't cheetin', you ain't tryin' hard enough."
