Rattling Noise-Just Started

[Weirdo_Joe]

The gas here is 85 for regular unleaded; at my gas station, anyways. And word for word, this is what the manual says- (note the last line, if nothing else)
"Use regular unleaded gasoline with a posted octane rating of 87 or higher. If the octane rating is less than 87, you might notice an audible knocking noise when you drive, commonly referred to as spark knock. If this occurs, use a gasoline rated at 87 octane or higher as soon as possible. If you are using gasoline rated at 87 octane or higher and you hear a heavy knocking, the engine needs service."
I've used 87-mid grade every time. I checked the oil in 3 different places, all appearing to be level, but just in case that was it. Good to know, though! That it's not "too much" I would describe the knocking as heavy, though, and ONLY when 1st idling. I am starting to think maybe the car was sold to me, knowing it had problems. I've wondered if my engine has full power, as it takes me 15-16 seconds to get 0-60 mph. On a stopwatch; not Traxx.
As for the rattling, I drove it a short distance today, and it started out REAL bad; I almost parked it again after backing up 3 ft. It seemed like a rattling-squeal today; a constant screeching, metal on metal sound, with a rattling, too. It kinda went away, after driving another 150 ft or so, still making noise when I turned in the lot, but almost gone by the time I reached the street. It started again after my 1st and only stop, but not near as bad. When I returned, it was gone. ???
I still have an 8am appointment tomorrow morning that I'll keep; sounds like something is wrong, and the old manufacturer's warranty w/ no deductible at all expires early april. I want my car to run like it should, by then.
Oh, and I have an LS sedan, and I'd say I'm a lil' higher than Denver-closer to the foothills-maybe 6000 ft?