I would be leaning toward something loose, see if the inspection cover or something is rubbing the flywheel, I wen through a puddle with a trans am before and the cover was not bolted proper and it slid enough that the flywhell wa rubbing sounded like the engine wanted to explode

 

That must have something to do with it. My only wonder is if that would make a consistent enough knock to sound like an internal engine knock. Or could it be unbalanced and have ruined the bearings...this sucks. Its going back to the shop on monday for them to inspect the job they did and I am going to bring that up.
On another note, what kind of T/A did you have? I just sold my '78 Hurst T/A to fund the disco.

 

had a 79 with 403 old auto posi shaker hood, 81 with 403 olds auto 4 wheel disc posi shaker hood, had 83 v8 5 spd, built a franken trans 1987 350 tuned port auto 4 wheel disc posi 92 nose fiberglass twin turbo hood and trans am ground effects, we still have a 79 10th anniversary trans with olds 403 auto loaded

 

That's crazy! I am after a 73 SD455 formula right now, if the disco would stop being broken. Made a 120 mile drive with her at about 85mph today and did just fine right till the moment i stopped in front of our destination. I'm almost 100% convinced this is trans related now. The engine is dead quiet at speed. Does anyone have a "shift shock" kind of feel when they let off the gas? I figure it may have something to do with the drive shafts but figured I'd ask. It does make me at least a little feel like i am driving a manual...

 

you need to get to that trans shop before parts start falling off or something

 

So, I took it to the trans shop today. Of course, she totally stopped knocking right up until the minute I was too far away from the shop to turn back around.
They were kind of less than helpful on the whole situation
anyway, I took a vid of the knock and how it doesn't go past about 1200 RPM.
Check it out here...
LR KNOCK | Flickr - Photo Sharing!

 

sounds like a blown exhaust gasket maybe where pipe and manifold meet, valve tap or knock would get louder and faster with rev up

they had to at least drop the y pipe to do the trans bet they lost or reused the old gaskets, possibly loose y pipe bolts

 

Wow, it would make a knock that loud!?!?!? That's crazy. I will go have some sketchy shop fab me up an exhaust then if that's the case. I do love Texas.

 

I does not sound loud to me, about the same as a valve tap, may just be video quality or my laptop speakers

a new exhaust is not the solution IF it is a leak replacing the leaking metal gasket is

 

When I redid my y-pipe, I didn't torque the pipe-to-manifold nuts enough.

I was alerted to the issue when 3 of em fell out and made the disco sound like a diesel school bus.