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This may sound stupid, but check your distributor cap. I’ve seen them “unseat” from the distributor due to hitting the firewall when the engine lays back during transmission removal/installs. Rotor hitting the cap while running makes one hell of a loud noise. This but me once and I’ll never forget how simple a fix it was after days of scratching my head trying to find it.
 

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I have a '70 that I put a 383 into, new flex plate and TC. Removed my old TH350, bought a freshly rebuilt one, and put it in this weekend. I now have a loud, consistent knocking coming from the upper section of the bellhousing that wasn't there before. Sounds loudest right where the engine meets the trans topside. TC bolts are the same as before and all tight. Housing bolts all seem tight. Flexplate doesn't appear to have any cracks (shouldn't since its brand new).
Anyone have any advice on where to go next?
Sounds like the flex plate on the torque converter. I've had this happen. Be sure you have the correct bell housing
 

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Not sure I am reading these two posts correct, but the engine will not start without the flexplate attached.

I had a knock once as you are describing, the flexplate bolts were loose.
 

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i pitched a driveshaft once on a second gen. when i put it back together it had a bad knock at top of bellhousing area. thought sure i had destroyed trans or hurt the motor. ended up being the distributor cap. still ran fine but sounded terminal. ck your dist cap
 

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Is this a new 383 build?
If so, my guess is one (or more) of the connecting rods is hitting the oil pan while the engine is running.
You may need to drop the oil pan and look for any contact marks on the pan (especially in the area where the pan is bolted to the block)... and if any are found, just slightly bang the area of the pan out with hammer.
put your hand on the oil pan while the engine is running. if anything is hitting, you will feel it
 
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