*** Bulletin number 02-06-01-022F (section 06 - engine /propulsion system) 11/30/2006 ***
It says in short................
This noise is caused by the relationship of the engine cylinder block, crankshaft journals, bearing inserts and the oil film. This relationship may allow a vibration that resonates through the cylinder block and is heard as a metallic ticking noise. The noise could be described as an irregular ticking or clacking sound, like random typing on a mechanical typewriter.
All Duramax diesel engines have this condition, however, some are more audible than others.
Testing and engine teardowns with this condition reveal no condition that would affect reliability or durability of this engine.
Dealers should not attempt to compare any customer vehicles exhibiting this noise with other similar vehicles as the noise is different form vehicle to vehicle.
This noise is a normal operating characteristic of the engine. This noise has no short or long term effects on the engine . Do not attempt repairs for this noise.
info numbers :
GMC 1-800-462-8782 USA
1-800-263-3777 Canada
Chev. 1-800-222-1020 USA