1995 Corsica V6 3100 issue(s)

First off: I'd like to say that I am not the first, second, or third owner of this car.
Second: I have no idea what has all happened to the car, all I know is the following:
1. The engine says 3100 on it, but the VIN number states that it is a 4cyl (So does the Government)
2. It's probably been in some sort of accident.
3. It has 247K on at least the chassis.

Here's my two main issues:
The biggest is that it makes a noticeable noise when it is idling and driving. It sounds like a light knocking and rattling that is coming from the engine. Not sure what would be causing it (Not well versed in automotive mechanics, currently in school for a degree in Secondary Ed in English and Comp Sci).
Our family mechanic did plenty of work on it before I got it, put a new alternator into it along with plenty of other work done. (It used to squeal pretty bad when I started it once before I officially "got" it) He's done some work on the breaks (they were completely shot when he started working on it) along with some other "major" issues, but hasn't done a complete overhaul on it yet (He's a mechanic educator, so he has plenty to do besides fix a college student's car)

Secondly: I want to fix my AC.
I bought a charge kit last winter (when it's on sale cheap) and tried fixing it this summer by charging, and it worked for a few hours and then was dead again. I'm hoping that it isn't too difficult to get the car's AC working again (School is 4+ hours from home, and home is 4+ hours from the female in my life, so AC would be nice)

Thanks for your time and consideration. I really enjoy driving the car. It ended up developing a large hole in the muffler, around the same time that the strap holding it broke off, so $15 at Menards for some dryer vent and hose clamps fixed that. It's also been through plenty before me. There's some evidence of a rear end accident, and the engine has been replaced with a 3100, but besides that, I know little about the car's past. All I know is that it runs and drives great.

If it is a rattle, I would look at heat shields, exhaust etc.
A/C. If it only lasts for couple hours after recharge. You got a serious leak. You need to add some dye and track down where the leak is. Most a/c issues nowadays, not a "cheap fix" since most of the time you have to take it to an a/c guy(he has all the tools, gauges)

If it is a rattle, I would look at heat shields, exhaust etc.
A/C. If it only lasts for couple hours after recharge. You got a serious leak. You need to add some dye and track down where the leak is. Most a/c issues nowadays, not a "cheap fix" since most of the time you have to take it to an a/c guy(he has all the tools, gauges)
I'll check it out later today, and upload a video to YouTube to give a better example of what I mean.

edit: http://www.youtube.com/watch?v=R15arY0kylk There's the video

As for the AC, the charge kit had a pressure gauge. After the cold air stopped, I checked the pressure again and it was still where it was before, which makes me believe that I either charged it wrong, or that the car vented it somewhere.
I will check again later this month when I get my hands on the kit again to see what the charge is at now, but I think it will probably be the same as before.

Thanks for the help.
One other random question: how hard is it to re-mount the driver's seat? one of the previous owners moved one of the rails, and now the seat sits just a little too far back in the car for my comfort. I'd love to be able to move it forward another inch or two, but it won't go any further, so I end up having to throw the spare tire behind the driver's seat in order to push it further up.

Also, does the Corsica come with a tilt wheel as a standard option, or was it something that needed to be added? mine doesn't tilt and I've always been curious if it was supposed to.