I am going to assume that you're in California, since they're the only state I ever hear anyone getting harassed for having loud exhausts. If so go get your exhaust tested for noise properly. Do yourself a favor and call the BAR to schedule a test, 800-622-7733.

Since I live in MA I am only aware of this from the May '04 issue of Sport Compact Car and Josh Jacquot's page 24 article on how many cars are actually legal and that an officer can't just make a judgement on his own as to what's too loud.

If you've already tried this approach and it didn't pass, then I guess you only have one choice.