I have the same issue both with an 8800GTX and a brand new 285 graphics card. There is some kind of IO interrupt or something hitting JUST as the animation stars.
It is very slight but very noticable and one of the things that can easily ruin the sleekness of the UI.
It may be specific to the nVidia cards? They practically all have the same driver
. I have other graphics weirdness with my nvidia cards with the Windows 7 driver that I never had before though (no texture animation for example, visible in games) So the driver will hopfully get better.