I owned a 2005 Prius for 4 years and almost 84,000 miles with never any fuel octane or engine clatter problem. I traded the '05 for a new '09 Prius on Jan. 17 '09 (because I can't stand the '10 Prius interior restyling). Now quite often while accelerating onto a highway, I hear slight engine knock or clattering sound. I am using the exact same brands and grades of gasoline as I did with my '05 Prius. I have taken it in to a local Toyota dealership service department and they could not find the problem or solution. My gas mileage with the '09 is comparable to the '05 (high 40s) and power seems the same. Can anyone give me some idea what is going on? It is not my imagination, I definately hear what sounds like low octane engine clatter. I am using 87 octane gas and the dealership said not to upgrade to higher octane as that might make matters worse. Help, please!