What is causing my exaust leak?

What is your question?

Ive had some flanges repaired. The exaust leak is in the engine compartment coming thru fresh air vent. Its worse during a cold engine start after sitting but can be noticed at all times. No engine lights are present. Is it a rich/lean problem? Is it an O2 sensor problem? Could it be a fuel control problem? A catilytic converter problem? Ive taken it to 3 mechanics and non can find the colpret...plz help.