Well I had a similar noise 6000 miles after my 2.6 was rebuilt. I park the truck as soon as i could pulled off the valve cover,loosened the head !!DO NOT RECOMMEND!! I did it anyway the head should be pulled off and all gaskets be replace ,lowered the oil pan,and then take off the timing cover. When I did that the tip off the driver side of the main timing chain guide fell off the main chain I replaced it. I blame the the rebuild kit i bought from rock well. 500 miles later the #4 connecting rod bearing began to knock !!This may or may not be your problem!!! When you do look in to it have a trustworthy mechanic look at it and make sure they flush the oil system to get any particules out the oil. I recommend you dont do it your self its rated 9.5 on my scale 10 being the hardest to do. Its no easy job unless you can turn a good wrench. It took 14hrs for me to fix my timing chain problem.Hope this helps a little. P.S. I did it inframe a little more tuff but saves time to me