I just got my '68 327 2bbl on the road. Everytime it gets above about 3000 RPM (don't know for sure - no tach)I hear a hard rapping noise that seems to come from inside the engine. I adjusted the rocker arm nuts as per the Chevy manual for fear of loose rocker arm, but no dice. When I installed the oil pump I didn't attach the nylon ring to connect the pump drive shaft. That wouldn't cause that sound would it? The engine is really brand new with only about 90 miles on it. Do I need to bust off the pan and install that nylon ring, or could it just be a dirty lifter? Wouldn't a lifter make noise all the time if dirty? Thanks in advance.

That nylon ring is what keeps the two shafts in line and concentric. You want that there, regardless of whether it's causing the noise you're hearing. 'Til I heard about the nylon ring, my immediate thought was - pinging. Don't know what kind of noise the pump shaft will cause, but eventually it'll be an ugly one.

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Huck
355 SBC powered '87 Chrysler Conquest TSi - 10.3:1, Vortec heads, XE 268H cam, HEI, eq. length shorties, Performer RPM intake, 750 Edelbrock (1407), T-5 and 3.54:1 posi indep. rear

Just went through this with my brand new 383. I haven't torn it down yet, but the general concensious is detonation. My tune-up shop (now former tune-up shop) set my initial timing at 16 deg and put it on ported vacuum advance. Not a good thing!
I got the same noises your hearing. Check your timing and retard it a little. Also you should be on manifold vacuum. Hopefully your noise will go away without too much damage.

You should be running about 10 degrees at idle. If its higher than that your total timing could be extremely high causing it to spark before the piston reaches top, pushing it back down and making the noise. It might just need to be timed.