Yousif

I have a 2004 3.0L Quattro. I have noticed a continuous thumping noise coming from my transmission or drivetrain(I believe). It happens when I am between 2,500 and 3,000 RPM in 4th gear only. When I let off the acceleration in 4th gear and in that RPM range the noise goes away. It doesn't happen in any other gear(a little in 3rd but barely noticeable in the same RPM range) or in any other RPM range. Any knowledge to correct this issue would be greatly appreciated.