2 things:
1) Check the heat shield around your cat. Mine was rattling at all sorts of random points - idle, low RMP acceleration, and around 2,500 RPM. I eventually got under there with some pliers and ripped it off. The seams were completely rusted out, except for one corner.
2) If you have a smartphone, you can pick up a decent bluetooth OBD2 scanner online and purchase the Torque app. Personally, I love it. The Torque app is a scanner AND a digital dashboard. Plus, you can look up the codes right on your phone and you don't have to flip through and books. Just my opinion.
Good luck and keep us posted.
1; I never knew there was a heat shield there -_- I'll have a poke around today.
2; sadly I have a windows phone so no Torque app from what I can tell. The app sounds really neat though. Sometimes I wish I didn't have to be different and buy something nobody has. Lol. Ill pick the scanner up soon, as I may have said. I want to try and do it within the next month.
EDIT would that be okay with one of you guys to take a pic of the underside of your car to show how low the exhaust hangs? When I go over some larger, more sweeping bumps, something scrapes along the road, from what I can feel. A pic from the rear would be best but it's entirely at your guys' discretion.
I guess I know what I'm doing once I have the funds:
Motor mounts- I'll get all 4 but do the sides and front for now (this'll end up happening in the summer most likely)
OBDII scanner- that's a no-brainer in my case. (I'll need to pick it up w/in the next few weeks)
My mom's all like "I just don't get why you spend that much money." It's my pride and joy. If I had 8k to throw away on a prepaid lease or something easy like that, maybe I might have done that. But 1) I don't and 2) I've loved how the P5 looks since I almost got one at the end of junior year of high school... Besides, I think I can justify the rebuild and doing all this work. Yeah, we don't have emissions or anything to worry about RE CELs and such, but I'd rather not have to look at that thing for my whole time owning her.
EDIT 2:
Pardon my multiple edits but I got a video of when the motor almost dies:
<iframe src="https://skydrive.live.com/embed?cid=50C06A9C62DCD437&resid=50C06A9C62DCD437%211078&authkey=AMBSRRc7rLXWbCU" width="320" height="240" frameborder="0" scrolling="no"></iframe>
I gave it a small bit o gas and then turned the heater on- maybe its the compressor firing up and that kinda kills the engine slightly? That wouldn't explain the flashing CEL though, would it? I told my pal and he suggested it could be a coil going bad perhaps? I tried holding it at around 1.5k and the "idle" fluctuated a small amount, 50 rpms at most either way. Besides that, I don't know if I'm any farther than when I began my journey...
<iframe src="https://skydrive.live.com/embed?cid=50C06A9C62DCD437&resid=50C06A9C62DCD437%211077&authkey=AG9orLnJ7Umowj0" width="240" height="320" frameborder="0" scrolling="no"></iframe>
That was what was left of my heat shield. And it stopped the rattle

I guess the other half is hiding somewhere in MI or something.