This noise may sound like a rod knock, but it is intermittent and it increases in intensity as the engine warms up to operating temperature. The "cackie" noise may by caused by pressure variation within the fuel rail and in virtually all instances is attributed to cylinder #8 (driver side rear). The "cackle" noise does not damage the engine or decrease the engine’s performance or economy.
SERVICE PROCEDURE
The "cackle" noise will be found early in an engine’s life due to low fuel pressure and/or the relationship of injectors and engine firing order, or anytime there is a drop in fuel pressure. If a mature engine develops the "cackle" noise, low fuel pressure is the likely cause. Before making a repair attempt, the following tests should be performed: Verify that the noise occurs with a hot engine, not cold.
Verify that the vehicle has been driven at least 20 miles since the last injector replacement. Air trapped in the system could cause a noise that sounds like cackle.
If the fuel pressure is less than 55 psi in either head, repair it first and re-evaluate the "cackle" concern. Repair any other engine performance concerns prior to attempting a repair for "cackie". Example: Low pressure concerns may be causing low power output and "cackle" and repairing the low fuel pressure may eliminate both concerns. Also, an engine ‘miss’ may be an indicator of valve train concerns.
If you have an Injector Performance Analyzer, determine if the "Cackle" is coming from #8 cylinder. Cancel that cylinder to determine whether the noise still exists. "Cackle" cannot be identified using the Cylinder Contribution Test.
Some or all of this information was provided by the Automotive
Parts Remanufacturers Association (APRA). For more information on
technical bulletins available through APRA call 703-968-2772 or visit www.AutoBulletins.com. 
