Hi there: My 2006 A4 Cabriolet is making an loud humming noise and this morning it had trouble starting. The noise occurs when I turn the key before starting the car and appears at times when driving the vehicle. It is VERY loud and appears to be coming from the passenger rear quarter panel.

Thanks so much for your help!

 

Sounds like it. Replace the fuel filter first in case it's just clogged up and causing the pump to overrun. Chances are though that you'll need the pump replaced.