For two months I thought that I had a loose heat shield or some kind of valve problem. Every time I revved much above 2500rpm the car produced a metallic clatter. Eventually it dawned on me that it might be pinging (which would be a first for me in any car) so I put about 4 gallons of premium fuel in when it got down to half tank. Amazingly enough, the clatter is nearly gone under load. Fuel economy is up around 5% over last week, too. What do we think? Carbon buildup? The car does have 91,000mi on it, and it probably doesn't see engine speeds above 3500rpm more than once every couple of months. Anybody have a favorite fuel additive to recommend to clean things up? Sadly, an "Italian tune-up" is impossible with a CVT without getting arrested.