Cadillac Repair: 91 cadillac sedan deville, cadillac sedan deville, accurate diagnosis

Question
I JUST PURCHASED A 91 CADILLAC 4.9 WITH 99,000 MILES IT HAS A LOW END KNOCK WHEN STARTING AND WHEN DRIVING. I CHANGED THE OIL TWICE WITHIN FIVE DAYS SINCE I BOUGHT IT. THE ONLY PROBLEM WAS IT NEEDED A CATALYTIC CONVERTER WHICH WAS REPLACED BY A REPAIR SHOP. I USED PENNZOIL SYNTHETIC 10/30. IT RUNS GOOD, PLENTY OF POWER, WELL TAKEN CARE OF BY AN OLDER GENTLEMAN WHO WIFE PASSED AWAY. THE CAR SAT UNTIL I PURCHASED IT. IT WAS WELL MAINTAINED BEFORE I PURCHASED IT. I AM THE SECOND OWNER OF THIS VEHICLE, WHAT COULD BE THE PROBLEM ................ PLEASE HELP AS I'M AT A LOSS.  YOUR HELP WOULD BE "GREATLY APPRECIATED".

Answer
A low end knock could be:

crankshaft MAIN bearing(s).

Piston(s) CONNECTING rod bearing(s).



Without personally having access to the vehicle and hearing it myself an accurate diagnosis is most impossible.



You can try isolating which cylinder is causing the noise by pulling  a spark plug with the engine running and see if the noise changes in intensity. Do this with each cylinder.