Ford Territory knocking

On a trip to Queensland in October 2011 I noticed a knocking noise while driving my 2011 SY Ford Territory uphill under load. I reported this at the 15,000 km service, but the dealer was unable to detect any problems. Before the next service the engine developed a pinging noise under light throttle at low speed, and the dealer suggested I use 95-octane fuel instead of the recommended 91. I have now been using 95-octane fuel since the 30,000km service with no noticeable change. The dealer also suggested using an additive to remove carbon build-up on the pistons as this could be a cause, but I doubt that carbon could build up in such a short time on a new vehicle to cause this problem. I asked the dealer if he could check the crank angle sensor, but he did not agree that it was a possible cause. I am concerned that engine damage could have occurred. Hope that you may be able to help.