Not sure why. Gas on effect makes it sound like a drivetrain or motor mount issue. I have same problem w/ 2003 Passat. replaced control arms, even axles. Check wheel balance & roundness. Good luck.

01.5 B5. Santorin. Original owner.
Several generations of mods later, my current setup is:

KO4's w/ AWE "headers" DP's & twin2
Latest AWE/GIAC "high altitude" program
RS4 clutch
AWE IC's
APR bipipe
710N
UUC Drivetrain Stabilizer
Bilstein PSS9000
18" RS4 wheels
UUCShort shifter
Brembo brake kit + SS lines all 4