Even if the engine was rebuilt, there could still be knock if it wasn't rebuilt correctly. Rod caps may not have been torqued correctly and have lost some clamping power to the journals, causing slap...or everything was assembled w/o use of assembly lube. There could be alot of variables inside that motor if it wasn't done right. I would take a look at your oil to see if there is any metallic flakes swimming around. Also, was the oil changed since the rebuild? The oil that is first used since the rebuild should be changed very soon after startup to flush remaining debris out. Maybe none of the above is your problem right now, but very well *could* be.
Harmonic balancer....take a peek at it and see if the rubber that is inside is starting to separate from the outer pulley. Actually, easiest way to tell if it is the balancer...take all belts off and start it, but don't let it run for more than say 10 seconds. That should be plenty of time to determine if your noise is in the bottom end or not. If it goes away, there is something in the accessory drive that ain't what it supposed to be (a/c, */c, etc.). If it is still there, either the balancer is shot...or something inside the motor.
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1993 SSEi - FrankenJinx - dead and gone.
1994 SSE (136K strong) - 180* t-stat,HD alternator, most unequipped SSE on the club, XM radio, Kenwood CD/MP3.
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