I've got 2003 GMC Sierra Z71 with 5.3 L (82000 km). I just returned from a long trek with my family visiting friends and some family members. The trip was about 8500 km and everything was great until I reached Nova Scotia. We stopped for an overnight break and next morning when I started the truck, it started making loud slapping/tapping noise. I shut off the engine and checked the engine oil level. The oil level was OK. I restarted the truck and the noise continued for about three-four minutes and then it disappeared. I did have a truck a while back with a noisy lifters and eventually would disappear as the oil worked its way through but this noise was much louder and would continue for a lot longer. I am begining to think that this is a piston slap and do not understand the cause for this event to occur. The noise continues every morning for a couple of minutes and then it disappears. On the short on/off startups, everything seems to be ok. Any input would greatly be appreciated.