Ive had a CEL for a little while, and I assumed it was more of the same stuff I have been dealing with all summer, I made an appointment with the dealer for tomorrow. Just a few minutes ago I noticed a pretty loud rattling coming from the exhaust at idle. Any ideas? And is this likely related to the CEL?

 

depends on what the error code is...exhaust noises (especially rattles) are usually attributed to a bad cat

 

So does that mean I should do my exhaust upgrade sooner than I planned?

 

well you're gonna need to keep the cat even if you upgrade, unless you do a full system. most people just do cat-backs.

 

I dont really understand any of this stuff to begin with. A little crash course maybe?

 

http://auto.howstuffworks.com/question66.htm that will explain the cat.

now for the full exhaust. A full exhaust usualy includes the cat/cats already in the kit.
a cat back exhaust system is a kit that starts from the stock cat and replaces everything behind it.

 

So how much is that gonna run me to fix? And what is the best way to go about it?

 

Check your metal sleeves (clamps). It's what's been a problem Sunset Audi hasn't taken care of the 8 times I've taken it in. They won't cover it because they think my rattling is a "Natural Characteristic." Good luck w/ whatever it is and gettin' the aftermarket exhaust...