C230RYDER

own a 07 c230 with about 31000 miles. three months ago started to hear this whisthin noise when im going up hill i mean the car is prob. on 3rd gear and it switch to 4th u can hear it.on high rpm at first the tech say it was spark knock then he changed it to the small runners switchin to the long runners later the techinal specialist say the vechicle is retarting the spark at high rpm so it does not bang in the next gear. test drove a car jus like it at the dealers lot didn't do it they say it bcause it has only has 20k on it but when it gets a little more miles on it they will all do it.