'03 A4 V6 3.0 44,000 miles - Upon acceleration, my A4 begins to vibrate noticeably at 20mph. As speed increases and gas pedal remains depressed, the vibration turns into a shaking. It is at it's worst around 50-60mph. When I let up on the pedal, shake goes away...depress and the shake comes back. I can also hear a faint knocking. Please help!