Hello all. I have spent the last 2-3 days searching this sight for my issue and haven't seen it. i purchased a 98 blazer with a locked up engine and installed a 97 engine that was supposed to be in good working order. the only difference in the engines that i could see was a little 2 wire sensor on the driver side head so i switched those out installed new valve cover and intake gaskets as well as a rear main seal, oil pan gasket and t-stat. after installing the engine i have a nasty knock/rattle that even with my stethoscope i cannot pin point where excatly it is coming from. im at my wits end and do not know if i need another engine or not when i had the pan off it looked clean inside. but i cannot find any thing. is there any thing i could have done during the swap or maybe the 98 flex plate differs from the 97 im not sure at this point. any help would be great.

 

Unless you personally inspect each internal part you may never know until you do. Lifters could not be working correctly. Rod pin could have excessive wear. Rod bearing could be worn. My advice is if you have the engine out take the one you wish to install apart, inspect the parts. Replace what is out of spec.
I do not know how to check out lifters however if the problem lies there after you have rebuilt and installed the old parts, access to them is easy enough without having to pull the motor again. The cost of parts is minimal and you will have peace of mind knowing what you have.