Cold start noises are common on the 5.4 and 4.6 engines. It's normal and no need for concern. I have not heard this often on the 4.2. My 5.4 did it for a while this past summer. I thought it was a lifter, but it went away after I started using Neutra 131.
I don't know why, either because the temp was getting colder, or the Auto-RX quieted my engine down and now I could hear the fuel injectors which were cleaned up with the Neutra and quieted down also.
As I recall, piston slap was an issue with the 5.4 engine only. Given my experience though, run some of Molecule's FP/131 brew after your rinse, clean the MAF, and change the fuel filter and you will be very happy with the results. I am.