sorry for the long post but here all the info. i got a 98 blazer 4dr 4x4 and i replaced the motor with a 99 blazer 2dr zr2 engine cause my old motor locked up due to main bearing spun. got the truck put all back together started it up and runs fine but there is an loud knock its a high pitch fast type knock. dont sound like my old motor with the bottom end going out. i checked where the sound was coming from and mainly where the motor and tranny meet but i can hear it at the top of the motor too. someone told me flex plate. well i checked the inspection hole in my trans bell housing and there is metal shaving all over the inside. i put a new flex plate on but i noticed that the zr2 flex plate that i took off the motor looked different than my old motors flex plate can this be the problem...... need an answer oil pressure is great knocking at idle and when i rev the engine.... doesnt go away

 

I've had a flexplate broken on my caprice once and managed to make a movie and put it on youtube.
my mech said that i was lucky to have reached the garage at all because the center has broken out of it.
Here,s the link to the movie I know that there is a difference between a balanced flexplate and a unbalanced flexplate. I think it has somthing to do with cranckshaft balance being internal or external. (correct me if i'm wrong)

 

Why not use the flex plate off your old motor? You know it worked before. Yea the wrong flex plate could cause an issue.

 

Cracked flex plate or a bolt head hitting the bell housing or something.
If it's at the rear of the engine up top I'd check the distributor & it's shaft. It may have gotten bumped & I know there have been issues with the shafts bearing going out & the shaft warping & that'll cause a high pitched rattle of sorts.

 

thanks for the replies i took out the tranny and inspected the flex plate its not broken but the weight on it the corners have been worn down so i looked at the back of the motor and nothing no rub marks but it had to been hitting something. i also checked out the tourqe converter nothing but there was metal shaving in the bell housing. so i replaced the flex plate with the original one that came with the motor im going to put it back together and ill send updates. i have done alot to this blazer and im not bound to give up on it. things ive done to it: new radiator, new/used tranny, ball joints, wheel bearing, new spider injector system, new vacum switch and encoder motor for 4x4, flowmaster exhaust, and a motor. this motor i got from the owner of the 99 zr2 blazer 4x4 ran great so its got to be something. oh and when i was moving the motor with it on the engine hoist the engine hoist fell on its side and the motor slammed on its back side busted the oil pressure switch and distributor switch i put a new oil switch and took my old distributor from my old motor and put it in it could it be possible that my ecm thats bolted to the coolant res be only good for my motor since the motor i got now is from a 99 blazer zr2????