03-05 Chevrolet Kodiak and GMC Topkick Rattle Grinding Noise 6.6L with ML6 Manual Transmission at Slow Engine Speeds - kw clutch fifth first flywheel fourth manual reverse second slow speed third

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Customers may complain of a rattle or grinding noise while operating the vehicle at low engine RPM’s usually below 1200 RPM. The noise is usually more pronounced in 1st through 3rd but can occur in any gear. Engine firing pulses lead to harmonics that sound out in the transmission. This noise is more prevalent in the C4500/5500 as compared to the HD 2500/3500 because a medium duty style flywheel and clutch assembly is used in place of the dual-mass flywheel and clutch assembly in the HD 2500/3500. All medium duty trucks will make this noise. Some will be worse, some will be better. Usually the more heavy duty, the components, the more noise. This issue is also exaggerated because the Duramax is extremely quiet for a diesel. You can hear the noise more than in a comparable vehicle.

Engineering has determined that this is a normal operational noise under these conditions and no repairs should be attempted. If the engine RPM is kept above 1200, this noise will be greatly or completely reduced. This is only a noise concern and will not have any durability issues.

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

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