My 94 Mark VIII does it too. 6000 rpms.. eeeeeeeee shift.
Like a slight, and very quick high-pitched belt squeek.
but this is good! That basically means your tranny is shifting so fast, that the engine rpm drop is almost instant, and you get a slack-drop in the belt. This also happens on radically cammed motors when they are revved. the idle recovery is so instant, the belts often flap for a second If you give it a double-rap of the throttle, you can get a slight, and quick 'eek"
I have a brand new belt, and idler pulley. I havent heard the little "eeek" in a while. But, I'm sure it will be back.
Oh, and BTW, our 97 does it too...it has a SCT chip, so the shifts are a little quicker than stock...
sleep tight...its fine.