Holden Commodore 2010: Knocking sound

Since purchasing my 2010 Commodore SIDI SV6 there has been a knocking sound similar to a 'pinging' noise between 1600 and 1900 rpm. It does not occur when the motor is cold and progressively gets louder and more often the longer the vehicle is driven. Initially the mechanics at the dealership were baffled and after a week in their hands Holden advised them to change the CPU. They also de-coked the carbon build-up. At first this seemed to do the trick but soon afterwards the problem re-appeared. Three or four attempts have been made to cure the problem, but to no avail. Each time after cleaning the carbon build-up this seems to help for a short while. There is no abnormal oil consumption. The vehicle otherwise drives normally with no apparent performance deficit but I have noticed that fuel consumption worsens to the point where I cannot get it to below 10L/100km on a trip until it is de-carboned whereby it returns its normal 8.5L/100km for a short time. The noise is quite noticeable to the point that passengers ask what is wrong with the motor. My concern is that the warranty is due to expire at the end of June and my fear is that this regular 'pinging' must be causing some damage to the motor and contributing to lessen its normal life before an expensive rebuild. The mechanics at the dealership have been great but seem to be stumped with the problem. All that they can advise is to monitor the oil consumption and keep them informed. What do you think? ?