If your exhaust has become louder, chances are that rust is eating away at the exhaust components making holes allowing the passage of exh gases out.
Inspect the piping, resonator and silencer for holes. There is a variety of exhaust packages available at low to high price ranges. I went with stainless steel BB Triflow cat back for $720 and couldnt be happier.
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No BMW at this time, eyeing a 91 318iS. 00 Saab 9-5, 89 Bronco XLT, 05 Kawi Z1k, 00 Ducati 748S Bip