i've had similar problems with this make of exhaust on fiat's, i replaced the system 2 times within 6 months for excessive noise (customer complaining) and to be honest she was correct.
All i got from manufacturer was never had any complaints before so send it back and get another one from supplier which we did but never altered anything.
In the end we just explained to the customer that apart from fitting a genuine fiat part there's not much we can do, she excepted that but not really satisfied but what can you do.