2002 Volkswagen Cabrio Exhaust system problems

I brought the car into a local dealer for maintenance upkeep and had the timing belt replaced, a transmission check and replaced the entire rear exhaust system. They also replaced the Catalytic converter and 2 oxygen sensors. This was all don in March and I have continued to have problems with the car, especially concerning the air intake valve. Now they're trying to get $500 more dollars out of me to fix yet another problem with the exhaust system, claiming there's a leak causing the car to sound awful until it warms up. I brought the car back twice after March only to be told it was a loose heat shield and they had repaired the problem. Frustrated, another mechanic took a look to fiind a leak in the front of the exhaust were the exhaust system connects to the catalytic converter. Is this something that would have normally been replaced with I paid to have the entire exhaust system replaced. He's claiming a leak in my air intake hoses, valve, etc.