Thanks for the information IanIt’s vibrating due to wear, grease sometimes helps, but usually it will need stripping and starting again, which is a roof out job.
Ian
The car is new to me 4 weeks old and its done it from day one, bought it as a project .The wife had a new one in 2004 and sold it 10 years later i wish she never had !You don't mention if this is how it always was or if you removed and re-installed the front glass panel for example. When I fully fixed mine, I had a similar judder with what you describe simply because it is difficult to perfectly align the glass panel to its factory position (there are too many degrees of freedom with the 4 screws holding it in place). However, with lots of trial and error the vibration/judder got better and now is gone completely after 1-2 months of opening and closing. In short, your panel may just be misaligned.
Of course it could be more complicated, as previously mentioned above, one side may be pulling more than the other side which is a sign of failing rails. The reason that the back glass panel is operating smoothly is because it is not linked to the (potentially failing) rails but rather "pushed" by the front panel.
Unfortunately that grease won't help much, if at all, sooner or later you gotta sort it out, or you risk one day it gets stuck open. Been there, ain't fun.The car is new to me 4 weeks old and its done it from day one, bought it as a project .The wife had a new one in 2004 and sold it 10 years later i wish she never had !
Then i saw this one at a place i was working started talking to the owner and the rest is history.
I'll have a go at what you said i've also ordered some proper Audi grease so we shall see