Hello, I am having an issue with my newly rebuilt motor. It is making a clapping/tapping noise under load and upon acceleration. It is not an engine knock.

I have a freshly rebuilt 350 Olds engine that I finally got running this summer. The engine was built a couple years ago and car has only been driven once since rebuilt engine was installed. Unfortunately, upon dropping the motor in the car and starting the car, I detected a very noticeable clapping/tapping sound coming from the motor. This is not an engine-knock, it is more like a loud rattling sound.
The motor is a 1970 Olds 350ci bored 0.060-over. It was rebuilt to stock specs along with an aftermarket cam. It has a 10.25:1 compression ratio. Timing is at 12 degrees TDC.

I have eliminated this noise coming from any belt-driven accessories (power steering, water pump, alternator, fan) and basically any other external motor parts. I have also changed the carb, fuel pump, distributor, gas, oil, and used colder spark plugs, all to no avail.
I had a mechanic come look at it and he said the noise was coming from behind the timing plate cover. I checked this and found the timing chain sprocket dots were aligned accordingly and there was no slack in the timing chain.

Does anyone have any ideas of what the cause might be? Any help is greatly appreciated.