Exhaust is making Loud Noise, Idles like I'm pressing on the gas

Does my Car Need a Tune UP? The exhaust system was replaced in the summer time with a sport muffler, and was not making any noise. Suddenly the noise shows up, I'm getting pulled over for noisy muffler, got ticketed. Took to a mechanic, he replaced a damaged washer but the noise is still there and he says its the sporty muffler tip. THATS NOT THE PROBLEM, something is creating the noise and I don't know what to do.