pirate is my understanding of the reason correct? i was told that once, when i had my Jeep those I6 4.0L engines were scary loud on a cold start it was like whap whap whap whap whap like it was low on oil for a minute.
The 4.0 was a noisy engine when it got old. It probably had "piston slap", like you described, where the piston fits a little loose till the engine warms up. They also had rod bearing problems and that was also noisy as hell. Damn near indestructible motors though. I remember during the cash for clunkers deal I was trying to kill off a 4.0l in a Jeep Cherokee. I had drained the oil and at this point was supposed to put a gallons worth of sodium slicate solution in the crank case. "Supposed to" was boring, and instead I throttled the **** out of it, while putting anything disposable that would fit past the throttle plate down its throat. It went on for over twenty minutes and by then it was over-heated, smoking, and making all sorts of interesting noises. It started to get boring and at that point I was recieving the "look" from my boss. I said **** it and poured the solution in it. The 4.0l took the death juice that had killed so many engines before and to my amazment started to run smooth as silk. WTF?? This isn't supposed to happen lol. After a few more minutes it finally stalled and wouldn't turn over. I had new respect for the ol' 4.0l. I had 454 tune ports that didn't even put up a fight, they'd go to sleep in a nano second after the death juice was poured.
There's alot of possibilitys where an occasional tick/rattle/lightknock might come from. Piston slap is one but usually that happens on worn engines. You might also get some start up noises from camshaft phasers, timing chain/tensioner, lifters, carbon build up. Engines have so many different mechanical (chaos) things moivng inside them I doubt you will ever find a completely tick/rattle/knock free engine. A little bit of occasional noise isn't something to freak out about.