I checked all the old plugs carefully last night and they are all intact from what I can tell. I'm hoping anything falling in there is not the case.

 

Get number one piston to top dead center and see if rotary button is pointing to number one spark plug wire on distributor cap .
Check coil .
Does it have points ? check those if it does

 

So I checked the timing today and everything looked pretty good. The knocking from what I can tell is coming from the valve covers, maybe the rods or valves. Here's a video link of what it sounds like before I pulled the drivers side valve cover. Once that was off I didn't see anything alarming to speak of so tomorrow I will pull the passenger side as well.
https://pin.it/zlbxhelkyldng5

 

I don't think we can view that unless we have some pinterest account?

 

Dang it. Sorry, is there a better way to add video?

 

Ok this video should work

 

Yes, that is super loud.

I have no ideas really other than that maybe previously there was a cylinder that wasn't firing that is now that you replaced everything. But, you said you pulled one plug at a time and there was no difference.

I might be wrong, but it seems like your vacuum advance is at an odd angle (which maybe means the whole distributor is turned wrong). I though they were supposed to be almost pointing straight towards the front like this...

 

That sounds eerily similar to some videos I found last week of what your engine sounds like when a spark plug is "blown out" and the cylinder isn't sealed all the way. (Like if the spark plug hole was open or leaking).



Here's a guy saying he had bad compression because of a bad spark plug and he had knocking because gases were getting past the plug.


Seems oddly similar due to the fact the you changed the plugs just before this happened. Like, perhaps one of them isn't sealing correctly.

 

Sounds like lifter clack it is valve train for sure though.

 

It sounds like something in the valve train. You could try to narrow it down with a stethoscope or just remove the cover and look for a problem.

 

He did already remove the cover and look, he said it looked normal

 

I didn't hear about him removing the passenger side yet, but if both sides are O.K. then he should use the stethoscope to try to isolate the source of the noise. That's definitely something that needs attention.

 

I would pull and check the plugs again and attach a nozzle and some clear vinyl tubing to a shop vac and see if anything fell in. (Twig, acorn etc...)

You might also check to make sure it is not coming from your PS pump or anything else that may have been bumped while you were working.

 

I've seen the vacuum advance on other 390 engines in almost the same position as mine. Prior to the spark plug change and compression test it was running good so I'm thinking the vacuum advance position is fine.

 

I'll give all this ANOTHER look... sigh