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becausethexo as others have said, just get it "fixed". Yea, the guy was rude but it sounds like it wasn't without cause apparently. If your car sounds "like a rocket ship" its just too loud for street use, but I'm guessing you already knew that as I'm not sure how you wouldn't have noticed it when you were purchasing it.
As others have said, bringing the police in would most likely just you get you both visited, and get you the tickets @
Tcoat outlined. Eventually you are going to end up having to fix it anyway, someone is going to complain to the police instead of you and you will get the tickets, or some officer will be sitting near you at a stoplight and ticket you.
I will fess up I just don't get the loud exhaust thing so I'm not the best judge. It serves no real purpose on the street except to say "look at me, I'm over here". I was behind a 2018 Mustang on the way to work today with fake Cobra stickers and what I'm pretty sure was close to straight pipes. It was loud, obnoxious and sounded like crap. An engine note needs to be tuned, and tuned does not mean loud it means tuned!