I have just bought a 1965 Stingray. 327/300 4 speed. Survivor. 51k original miles. It hadn’t been driven much recently. After driving until the engine is warm, there was a knocking in the upper rear of the engine. If I let it idle a few minutes the knock goes away. After getting it warm again, it returned. A stethoscope revealed its both inside the distributor cap and out of the shaft of the distributor. The noise is barely audible anywhere else I scoped. Upon checking, the distributor hold down bolt was not secure (you could adjust the advance and move the distributor without a wrench). I tightened it. The noise is constant now. Grinding and loud knocking at the distributor. Engine runs fine. Flattens out under a load at 4500 rpm, though. I can’t find anyone online describing the same thing. Any ideas??

 

Turns out the tach drive gear was broken. That was causing the noise.
New distributor solved the problem.