I did the "open-the-mounting-hole" and the "advance mod" on a new NAPA CPS. After I replaced it and started the engine I heard a not-so-loud buzzing sound for about 10 seconds then the noise faded away. Engine ran fine. I was thinking perhaps there was slight contact between the CPS and flywheel. I had also put some duct tape on the plastic cover that the CPS fits thru to keep things from falling thru so maybe a piece of that got shoved thru the hole during installation and was rubbing the flywheel.
As a side note, replacing the CPS on a stock '89 MJ 4.0 with BA10/5 is a pain in the tuckus! Detaching the front driveshaft from the front diff and swing it out of the way gave me some additional elbow room. When I did the advance mod the plastic housing developed a faint hairline crack. I encased the plastic in epoxy it before installation. Hope it doesn't cause future problems.