Okay very long story but ill cut it short, have a series 1 3800 supercharged, plenty of mods, had a 95 block in my 93, spun a main bearing, gutted my spare block and switched over the crank, rods, pistons etc. lasted maybe a week and my oil pressure switch failed and boom spun bearings

This time I got a 93 Park avenue ultra, heard it run, ran great had a little noise in the belts but the whole car was $400 with 114k

pulled motor, swapped my ported heads, etc regasketed the whole thing, got it in and running, had a clatter to it, replaced the cracked harmonic balancer and it helped, now theres a rattle under the throttle body and the whole engine shakes at idle, clatters bad at 1500 rpm, anything over that and it runs good.
Is this a bad balance shaft bearing? Ive had the engine out and in 3 times in the last 3 months and spent countless dollars on gaskets etc.
I do not want to pull it again if I dont have to
Has anyone experienced this before, and does the motor need to come out to replace the balance shaft bearings and or the whole shaft assembly

you check compression? look at the bearings? push on the lifters and see if any are loose?