I wonder if something like this could be helpful:
http://www.4wheelparts.com/Lift-Kit...ar-Pad.aspx?t_c=1&t_s=399&t_pt=4498&t_pl=3845
Also, here's a TSB for leaf spring noises. Maybe the noises are related and perhaps your dealer would be willing to try this out for you???
Document ID# 1999324
2004 Chevrolet Chevy K Silverado - 4WD
Subject: Rear Leaf Spring Slap or Clunk Noise (Replace Spring Inserts) #03-03-09-002B - (07/23/2007)
Models: 1999-2008 Chevrolet Silverado 1500/2500 Series Pickups
2000-2008 Chevrolet Suburban 2500 Series
2002-2008 Chevrolet Avalanche 2500 Series
2003-2008 Chevrolet Express 2500/3500 Vans with 8500 GVWR (RPO C5F), 8600 GVWR (RPO C6P) or 9600 GVWR (RPO C6Y)
1999-2008 GMC Sierra 1500/2500 Series Pickups
2000-2008 GMC Yukon XL 2500 Series
2003-2008 GMC Savana 2500/3500 Vans with 8500 GVWR (RPO C5F), 8600 GVWR (RPO C6P) or 9600 GVWR (RPO C6Y)
This bulletin is being revised to add model years and update the Parts Information. Please discard Corporate Bulletin Number 03-03-09-002A (Section 03 -- Suspension).
Condition
Some customers may comment on a rear leaf spring slap or clunk noise. This noise is most apparent when the vehicle is operated over irregular road surfaces.
Correction
Important: DO NOT disassemble the main pack on the leaf spring or remove the bushing from the spring.
Verify that the rear leaf spring inserts (1) are present on the leading and trailing ends of the overload spring (second stage leaf). The insert may have broken apart from wear and fallen out. If the inserts are not present, install new ones.
Important: On some models, the vehicle may have to be raised to allow the springs to separate enough so the insert can be installed.
Parts Information
Important: Technicians should refer to the Parts Catalog when ordering the insert.
Part Number
Description
15604067
Insert, Rear Spring Leaf
15247030
Insert, Rear Spring Leaf
Warranty Information
For vehicles repaired under warranty, use:
Labor Operation
E5168
Insulator and/or Insert, Rear Leaf Spring - One or All - Replace
Use Published Labor Operation Time
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
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Document ID# 1999324
2004 Chevrolet Chevy K Silverado - 4WD
