[Jim Walters]

Check timing at 2000 RPM maximum advance should be no more than 25°.
Smoke test for possible vacuum leaks.
Check mixture strength, if carbs set too lean pre-ignition will result.
Check compression, heavy carbon buildup in combustion chamber will cause pre-ignition, more likely in early 9:1 engines.
Test drives each time with different plug lead off spark plug and plug wire grounded. Plug wire must be grounded out to prevent flashover in cap or rotor causing damage. This may pinpoint which specific cylinder is causing the issue. Do this by removing plug leads from cap one at a time and have a short well grounded high tension wire near the cap that you can plug into cap socket that plug lead is pulled from.
Those tests should help pinpoint the fault.

 

[Jim Walters]

Isolating cylinders one at a time on test drives should help determine if it is a gudgeon pin bushing or connecting rod bearing. Both of those would give a lower pitch sound than pre-ignition pinking would.

 

[Jim Walters]

That doesn't sound like an internal engine noise to me either. One possible cause that has not been mentioned is the torque converter bolts. I have seen those loose on a couple cars and they will make a rattling sound. I had one Shadow towed into the shop a few years ago where two of the bolts had come loose and worked their way right out, catching the lower bellhousing dust shield and dragging it into the space between the torque converter and starter crumpling it up into that space. It made a horrendous sound. I would check those and if all OK there go to the fan belts next one by one, testing each time one is removed until you can test with all off. When you rev the engine, does it torque over to one side a lot indicating soft engine mounts? Probably not the issue because the fan hitting the rad shroud would be pretty easy to see and hear. As Rob says it could also be a loose baffle in a muffler or a bad exhaust leak, your recording sounds more like that rather than an internal engine rattle.

 

[Jim Walters]

Before you do the crankshaft seal mod have you checked and cleaned the flame trap? Neglect of it is often the cause of many oil leaks.