I found a company that addressed the issue. Subaru released this info:
The infamous "piston slap"
Summary
Description: Clicking, tapping, or knocking noise eminating from the engine after a cold start. The noise lasts until the engine is warmed, usually 5-15 minutes. The sound may be heard in the passenger compartment.
Affects: 2.5L DOHC engine, particularly from 1997 to 1999.
Diagnosis: The pistons are engineered for operation at temperature. In an effort to save weight in the engine, the piston skirts were shaved down. When the pistons are cool, however, they do not have sufficient contact with the cylinder to keep from 'slapping' or rattling against the cylinder sides. At temperature, due to thermal expansion, the pistons match up properly with the cylinder and do not rattle.
Reported solutions: In some cases, SOA has rebuilt the engine short block, while in others SOA prescribed "countermeasure pistons" with longer skirts that do not slap. Most dealers will initially state the sound is "normal."
Detrimental effects: SOA has said that the noise causes no engine problems or abnormal engine wear; Subaru engine expert CCR Engines of Colorado has said that the engine may experience a 10% reduction in life.
The only thing that i have been told from anyone from Ford is that the hypeurtectic (sp?) pistons expand and contract more than any other material. And that's what they SPECULATE about the cause of the noise. of course nothing has been officially addressed.
gary