L28 engine knock

[toto]

First, I apprieciate all the help this forum has provided over the last few years. Finally got to drive the Z. It ran and felt great. Sounded better than it ever had. At the last turn home, I downshifted into 2^nd and about a second later there was a loud knock sounding like a rod that quietened while coasting down to a stop in about 50 yards. Now I have an engine knock. I can't say I can really locate the noise. It sounds like cylinder 5 and with a screwdriver as a stethoscope, I can barely hear a knock near #5 at the head. I can't hear any tapping at any other location. The engine had about an hour on a complete rebuild. It looks to be oiling properly. The engine is turbocharged but the throttle was closed when the noise started. The knock is intermittent at idle, seems to be at cam speed at a few more rpm's and seems to double in speed with more rpm's. I've checked for missing lash pads and rechecked valve lash. Nothing apparent in the valve train. I've pulled spark and injector plugs and cannot detect any change in sound. This would point to the top but maybe I'm not very good at diagnosing noises. Noise is not from the water pump and I have checked for a blown exhaust gasket. My guess at this point is either a piston is striking something or I have rod damage. I can't imagine anything suddenly changing in the top end without leaving some evidence. At this point, I'm reluctant but ready to tear back into the engine. I would probably start by removing the head and then look at the lower end.

Any suggestions of what this might be or advice on how to proceed. I've owned this car for 29 years and I'm determined to drive it a while longer.   

[toto]

It turned out to be a #5 piston strike.  Obviously a result of my poor welding ability and a bead of aluminum breaking out of a dark corner in the intake system.  Hopefully, there is not too much damage.  I guess this means when you have checked 10 times, check again!