Alrighty, I'm trying not to be one to open up old posts, but I finally got a resolution to this thread's issues so I figured it would be best practice to post what happened.
To recap, I originally posted asking about two issues: Bad O2 sensors and vibration/rattling from the front. The O2 sensors were fixed quickly when I had the catalytic converter replaced, and they still are functional to this day, but the vibrations went on for ages. It went through two state inspections and one other mechanic without finding anything amiss.
Well, there certainly was something amiss. Every time I accelerated, turned a corner under throttle, or went over rough road there was a distinct vibration/rumble from the engine bay that turned into clattering pounding at its worst. The one mechanic I had it at was so sure it was a misfire (despite no OBD codes regarding that) that he spent two weeks going over the fuel system, but of course even after a fuel injector replacement for the #4 cylinder it still remained.
So today I decided to bite the bullet and take it to another mechanic with two new transmission mounts from RockAuto in the backseat with the instructions to replace them as he saw fit. I got a phone call three hours after dropping the car off; it was him telling me exactly where the vibrations were all coming from. It was not the CV axles, it was not the vacuum hoses causing rough running, it was not failing ignition coils, a loose supercharger, a bad supercharger coil, or bad spark plug/plug wire combo. It wasn't even those blasted engine mounts I had purchased and given to him.
THE TRANSMISSION, LADIES AND GENTLEMEN, WAS BEING HELD ONTO THE ENGINE BY ONLY 1.58 BOLTS!!!!!
Honest to Pete, he asked me right away if anyone had ever worked on the transmission before, to which I replied "Yes, the previous owners had it replaced at 80000 miles or so". He said whoever did it didn't tighten the bolts that held the transmission to the engine and sub-frame properly, and all but one were completely missing, while one other bolt was broken off inside the engine block! Out of six total, only one actual bolt was holding the whole drivetrain together! The source of vibrations has been the fact that the darn thing has been flopping around inside the engine bay for the last four years at least, growing worse as each bolt loosened and fell off. It got so bad that he stated one of the "ears" that the bolts go into had broken off, leaving nothing to mount with. Needless to say, he promptly fixed the issue for a reasonable price. And Holy Jiminy Christmas, does she feel like a Buick now! She is Smmooooooooooooooooooooooooth! It feels like there is no engine in there when driving along, and the screws don't back out of the dashboard every time I accelerate either!
While I was never had the chance to apply it, I want to cordially thank everyone on Buickforums.com who gave their two cents about the above problem. It was greatly appreciated and very informative to me. I now know more about the setup and analysis of my car's front end than any service manual could ever help with. And while my incident is (hopefully) an isolated one, I imagine there are many Park Avenue owners out there with vibrations that will find your information very useful, especially in one concise thread. Thanks again for your help, and keep on (smoothly) rolling!
NOTE TO MODERATORS: Please append this thread's title to include [SOLVED] so that readers may know that there is a conclusion to the OP's topic.
