Mystery knock
 

I had a acquaintance stop by for a oil change a few days ago with his recently rebuilt motor by a reputable source. Everything was done right from crank bearings out.
This is a stock 2.2 T with PMO's. Engine runs great with no oil leaks.
Adjusting the carbs I brought the idle down to about 600 rpm. At that point there was a distinct knocking noise, that sounded about half of crankshaft speed. As soon as you raise the RPM's past 850 on the way to 900, (spec), the noise is gone.
Engine oil pressure is great, even hot at idle. If you never dropped the RPM below 800, you would never hear it.
Now, this is not the first time I have heard this exact noise at the low RPM. I have heard it on other mag case engines though the years.
Every clearance was checked during the rebuild as I know who did it.
It is not a ticking sound, it is a subtle knock. I told him not to worry about it.

Wondering anyone else has heard this sound before.
I set the idle at spec, 900+- 50 RPM, and that was the end of it.

Sounds like possible intermediate gear lash or excessive crank end-play. With the age of these engines, I would expect these things...especially at such a low RPM. If there are no other noises present while driving, like a light knock on trailing throttle and there is no debris on the magnet, I wouldn't be too concerned.

Thanks for the reply Aaron, No other noises at any point. As you said, these mag cases are 50 years old now.

Good call, my thoughts as well.

One quick question: how do you know it's 600RPM? Generally the stock tach marks 1000, then 800?

My timing light has a tach in it, with digital read out. I would not trust a 50 year old tach to be spot on.