Starfire Tach Revs, But Engine Has Issues, Maybe

Well,  I got the necessary supplies to temporarily mount the Starfire tach to the engine test stand and wire it to the engine.  Once again, a weak battery caused me to have to take time for a charge, then we were off to the races.  The Starfire tach reacted to the pulses from the coil and it looked like this:

You can see that the tach is reading just short of 400 rpm, which is great, that means that it runs. The problem, though, is that the modern tach that was running at the same time read the following:

You can see it is at 900 rpm.  So a slight mismatch, but this was not unexpected.  The Starfire tach was for an 8 cylinder engine and my 235 is a 6 cylinder.  There are adapters that will make the tach read the 6 correctly, so I'll have to look into one of them a bit later.

For now, I've possibly discovered a new problem with the 235...I was revving it a little higher than I normally do when I'm running it on the test stand and I heard a new sound.  Not a good sound. It was a "tinny" metal on metal sound.  The sound went away when I backed off of the revs and returned when I revved it again.  So clearly, it is rpm related.  I checked around the valves and did a cursory check of the engine side cover, but nothing jumped out at me.  I'm asking on the Chevy truck forum that I belong to (www.Stovebolt.com) for help in trying to figure it out.  We'll see what we can come up with and I'll keep everyone posted.