Here is a copy of the GM bulletin:
"A random ticking noise may be heard in some LB7/LBZ's with the engine running at idle speed and with the coolant warmed to a temperature of 70 degrees Celsius or above. An Irregular metallic ticking/clacking sound like random typing on a mechanical type writer, can be heard at the lower rear of the left front wheel house. Some customers may also hear the noise inside the cab as well. This irregular ticking/clacking sound can be heard between idle and 1500 rpm. Above 1500 rpm the sound goes away, but returns once the engine is operated within the idle and 1500 rpm speed range.
The sound is caused by the relationship created from the engine block, main bearing crank pin, bearing inser, and oil film which generates a vibration that resonates though the block and is heard as a metallic ticking noise. This noise is most audible on vehicles that have engine main bearing tolerances that are closer to the minimum main bearing clearance specification. All Duramax engines have this condition, however some are more audible than others. Tolerances between nonaudible ticking engines and audible ticking engines are differences in clearances measured in microns.
Isuzu advises that they have experienced this condition in other diesel engines they manufactured over the last 20 years. Other manufactures of diesel engines such as Hino, Mitsubishi and Nissan also report similar noises in their engines. The LB7/LBZ bearing manufacturer has investigated this noise in the LB7/LBZ and state that no reliability or durability concerns are due to this condition.
General Motors Noise Vibration and Harshness group and Isuzu of Japan Engineering have done extensive testing and agree that no reliability or durability of this engine is effected by this issue. GM engineering has run a ticking engine on a 1200 hour dynamometer durability test and has also agreed that the noise condition does not effect reliability or durability of the LB7/LBZ engine.
The noise is a normal operating characteristic of this engine, It has no short term or long term detrimental effects on the engine. Do not attempt repairs for this sound."