I've got about 2,000 miles on a rebuilt 1936 Pontiac 6 flathead motor. From the beginning, it has made an odd noise when revved in a certain rpm range. It is coming from the front of the engine, sounds like a knock. I know it isn't:
• a generator bearing;
• the water pump or fan;
• engine bearings, pistons or valves;
• the inner oil splash pan;
• anything loose on the car that might vibrate (gas line, etc.)
• the distributor.

I'm wondering if it might be either the harmonic balancer or the timing chain/gears. The balancer is original and worked fine before the rebuild. The new timing chain was new, but the timing gears are original (impossible to find NOS gears). I've never had a problem with either of these, so I don't know what they sound like when they are not functioning properly. Or maybe its the engine mounts?

Any suggestions/advice most welcome. Thanks.

If you have original engine mounts, I would check to see if they are too hard or compressed. They may not be absorbing vibration...

The new timing chain may not be exactly the same spacing as the original. My '28 Gardner had thrown the chain many years ago, which led to its lengthy 35 year "storage", (along with broken pistons). After rebuilding and a new timing chain, it too ran noisy. My father-in-law who owned it at the time figured out that there was a poor fit on the old timing gear with the new chain. He calculated that the modern links were a bit longer than the old chain, and cut a new gear a few thousandths over for the correct mesh. End of noise. I hope this is not your problem, as there aren't many machinists around who can do this kind of work.