I might be a little OCD, but I can't stand my car to make noise! My 2003 Cooper S has been squealing pretty bad, especially when the ambient temp is hot.

Is this a common problem? I saw a couple other posts about this but they didn't really cover any sort of diagnosis/fix. Is it a belt issue or a pulley issue? The car is stock except some intake/exhaust so I don't think it is misalignment. Any help would be appreciated.

If your car has more than 40K on it and the belt has never been changed, start with a belt replacement. There a number of possible causes but the belt is the most likely one.

+1

Alright, I might just have to put a reduced pulley under there when I swap the belt. Thanks.

Good to see that you're thinking straight!

along similar lines- i have a 17% sc pulley, 0% crank pulley, and a new belt installed for about 6 months. lately there's some belt chatter/noise. does this happen often? should i replace/tighten the belt?

You can't tighten the belt. You might want to check the alignment of the pulley...see if one of the fasteners is coming loose. Belt size should be a NAPA 6035. Not sure what the Gatorback size is.