i don't think it is normal but my driver side widow has done it since day one and i currently have 42k miles now......some of my friends 99+ mustangs do it and others don't.....
My '03 does it too, and it seems to me that it should not. I believe the "thump" is the window hitting the stops, but if working and adjusted right, should it not stop just before that? In other words, I thought that the window would hit the stops only if something went wrong.
well 2 weeks ago i had to get mine replaced. it would thump way down and then when it was rolled up it would roll up too far. so i couldnt close the door when i got out. the construction of the powerwindows on the mustang simply cant be glued on once they come undone.