2004 Jeep Wrangler engine knock

I have an 04 wrangler inline 6 with 90000 miles. When I start the Jeep it knocks and then goes away after about 15 minutes. I change the oil every 3000 miles and put a good fuel injector cleaner in the tank. Oil pressure is good and steady, power seems to be good and strong. My question is? Should I be worried about it
and what could be the culprit to this problem.

Thanks for any Info on this. DARREN.