I did a lot of searching, but only found threads about rattles on cold starts...

So my problem is I have an '03 Coupe 6MT and whenever I have a shift that's not so smooth, there's a rattling sound that comes from underneath the car. It sounds like it's coming from right under where the driver's seat is. I don't really think the sound has anything to do with the transmission or clutch or shaft or anything because it only happens when the car sort of jerks(i just recently got it so still perfecting my shifting); never happens when it's a smooth shift. The sound reminds me of if you had a bolt with a washer on it and shook it around.

Anyone know what this could be?

 

Heat shield most likely. Do you have any after market parts?

 

Previous owner said he had a stage 2 clutch put in like a month ago, but other than that, everything is stock.

 

most likely a loose heat shield.

 

Awesome, thanks for your help! I'll get that checked out.

 

Another vote for heat shield assuming you're having the same problem I did.