I took it (2001 F150 4x4, 4.2L V6, Auto, 73k) to a Ford dealer at lunch time. Had a service writer listen to it. Of course it didn't knock when I started it cause it was warm. But he said it sounded fine. Piston slap is common I guess, but evidentally much less severe with synthetics.
I think I'll just cut my rinse cycle a little short (1500 miles) and go back to M1.
You probably had some gunk holding something together! Just kidding, I had an old T-Bird that knocked for about 5 seconds on startup, turns out it was way to much wear on the main bearings. But it had 130K on it AND had coolent in the oil all the time, so it had an excuse!
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.
After an AutoRX application (during it ran great) I'm in the rinse phase using 5w-30 Penzoil. The truck normally is fed a diet of 5w-30 or 10w-30 synthetic (mostly M1), which was in with the AutoRX. Now during the rinse I'm getting noticably more vibration from the engine at any speed, and today while pulling out of my driveway, got a loud knocking sound that lasted for about 5 seconds. Never heard that before! Now I'm sure this ins't due to the AutoX, but maybe the dino thats in it now. Could someone tell me if I'm just being paranoid or if I have a real problem?