so the dealer does agree there is a rattle?
try this, using a shop rag stuff it in behind the dash and drive it
is the rattle still there? if so try a different location under the dash
doing this hopefully you could find the location of the rattle
are you sure that something did not fall down in the windshield defroster vent?
This was from years ago who knows maybe GM still has this issue.
Bulletin No.: 00-02-35-003N
Date: March 26 2008
Clunking Noise Under Hood and Can Be Felt in Steering Wheel and/or Steering Column (Replace Upper Intermediate Steering Shaft [I-Shaft] Assembly)
2002-2006 Cadillac Escalade Models
1999-2007 Chevrolet Silverado Models (Classic)
2000-2006 Chevrolet Suburban, Tahoe Models
2002-2006 Chevrolet Avalanche
1999-2007 GMC Sierra Models (Classic)
2000-2006 GMC Yukon, Yukon XL Models
2003-2006 HUMMER H2
Attention: This Service Bulletin DOES NOT include Mid-Size Utilities such as Buick Rainier, Chevrolet TrailBlazer Models, GMC Envoy Models or Oldsmobile Bravada. Refer to Service Bulletin 02-02-35-006A or newer for Mid-Size Utilities.
This bulletin is being revised to add diagnostic information (refer to Diagnostic Tips) to check the lower steering column bearing as a potential source of the noise. Please discard Corporate Bulletin Number 00-02-35-003M (Section 02-Steering).
Some customers may comment on a clunk-type noise coming from under the hood that also can be felt in the steering wheel. These conditions may be more noticeable when turning at low speeds on rough road surfaces.
Use the information below to help diagnose the source of the noise.
The lower steering column bearing may create the same noise as the intermediate shaft. Before replacing the I-shaft, verify the noise isn't being caused by movement from the lower steering column bearing. Check the bearing for movement by pushing up and down on the I-shaft where it attaches to the steering column.
Frame Snap and/or Popping Type Noise - A frame snap or popping type noise can be duplicated on rough or smooth road surfaces with steering wheel input to the left or the right. This type of noise can be HEARD and is typically louder with the windows rolled down. For additional information refer to Corporate Bulletin Number 03-08-61-002F or newer - Snap/Popping Type Noise Coming from Front of Vehicle (Remove Front Crossmember Change Fastener Orientation).
Intermediate Shaft Clunk - Intermediate shaft clunk is heard and FELT in the steering wheel and/or steering column area typically while driving on rough road surfaces with steering wheel input.