I have a 2005 Sport Trac with 131k miles on it. It has been maintained very well, including transmission service every 50k. The truck has a Class III receiver as well as an external transmission cooler, but honestly it has not pulled much of anything during its life. Over the past few weeks I am noting an odd knocking noise when the truck is going into or coming out of overdrive. I notice it under light acceleration. I can hear the noise, but I do not feel any difference, no shudder, and nothing noticeable on the tach. I have scanned the system using Forscan and no codes are stored. Is this the beginnings of the torque converter failing? Transmission fluid looks great, pinkish/red, no burnt smell, and no particles noticeable when I pull the dipstick. Any thoughts appreciated.