Yours would be one of the earliest I've heard off but it sure sounds like your intermediate shaft has gone dry. It seems to affect all GXP'* in time, and other trim levels as well. I would take it in and mention the shaft. Most know the condition already.
1. STEERING Bulletin Number: 010232001F
Bulletin Date: FEB 2006
Vehicle: 2005 Pontiac Bonneville
CLUNK FELT/NOISE HEARD FROM STEERING COLUMN, STEERING GEAR AND/OR FRONT OF VEHICLE DURING TURNING MANEUVER AND/OR STEERING WHEEL ROTATION. ( NHTSA ITEM NUMBER - 10019516 )
There is also a good thread with a sticky on it in the 2000-2005 section of the forum http://www.bonnevilleclub.com/forum/...ic.php?t=41677