Adurham, I also have an Quattro, but an A4, that has the exact same symptoms at the exact same speeds. Starts around 9 and goes away 24-26, very distinct thump thump that you can hear and almost feel. It changes with speed but not direction or loading. Replacement of the front bearings and shafts, moving tires around etc has not resolved it. Put it on the lift and find no obvious issues with driveshaft, axles or rear wheels etc. Yours is the only net post I can find about another Quattro with these symptoms so hoping you or someone might have an idea. Did you get any resolve with your noise issue?