Ford Fiesta 2010: Pinging under acceleration

My Ford Fiesta two-door 1.6L has been pinging under acceleration ever since I bought it six months ago. I pointed this out to the dealer at the first service and was told that everything was OK, but that they had been having similar complaints about the quality of petrol! I mentioned that I had tried different brands and levels of petrol, but that the brand made no difference, and that the pinging was nearly non-existent when I used 98 unleaded, The reply was vague at best "we'll look at it at the next service". I have since taken it to my old independent mechanic and he has checked the plugs, filters, electronics etc without any luck. I bought the car after reading all the good reports and also for its low running costs, but it's getting annoying now with the noise occurring even under moderate acceleration. Have you any thoughts as to what may be the cause?