Was climbing a steep hill at 35 mph just before truck down shifted to 2nd, I was hearing a pinging sound. Sounded like only I cylinder was doing so. No idea what it could be. I am currently running a 1/4 tank of Fppf fuel injector cleaner. Is this a injector noise. No knocking just a slight pinging. Thanks

 

This sounds to me like the torque converter was locked, the engine was lugging, then the TC unlocked - but I doubt you dropped a gear.

When the TC unlocks, it slips - allowing the engine to spin up to keep better RPMs for the load. This give the impression of a downshift, but you're likely still in 3rd at that speed - unless you stomp on it.