BrightonBonneGXP. it sounds like your Intermediate steering shaft is going.
heres the link to the discusion in Northstar section
And theres a sticky about it in the 2000-05 section. It'* a common problem and get it in to the dealer while it'* still on Mr. Goodwrench.
Here'* the TSB# and desciption
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 )
All known TSB'* , numbers for dealer reference and descriptions are listed in the Northstar General Info sticky at top of page. Whenever a problem arises, thats a good place to start.