Idle Noises

Last month I came to a solution for my car dilemma. The local(ish) Mazda dealer was selling a used Mazdaspeed3 in great condition at a good price. After living with this turbocharged hatchback for a month, I can report that it is as awesome as car magazines would lead you to believe it is; it's a bit like hearing your favorite song come on the radio, followed by a comforting announcement that all the lousy songs have been banished from the station's library. I know in my soul that the people who engineered this car love AC/DC and loathe O.A.R. The main change that leads to this spirit of aggressive excess is the turbocharged, direct-injection 2.3 liter engine. Once the turbo spins up (3,000 rpm), the 280 lb-ft of torque carry the car along nicely. Horsepower is limited to 230 in first gear to reduce the risk of torque-steering into a cyclist or pedestrian. Once third gear hits, the full 263 HP become available. Zero to sixty takes a little over 5.5 seconds, the quarter mile is covered in about 14.0 seconds at 100 MPH, and top speed is electronically limited to 155 MPH. This bar stool trivia is all well and good, but there is one issue with my new car that been been vexing me. When the car is at idle, usually during its compulsory turbo cool down period, the engine rattles. This noise is enigmatic, but I have a few theories. Theory one: a motor mount is worn out from excessive hooliganry by the previous owner. Since the car found its way onto the dealer lot via a repo due to non-payment, I have concluded that the old owner must have been spending his (would a woman actually buy this car? Send your thoughts.) money on car maintenance and 91 octane fuel. In addition, if this mount were heading south, my research has led me to believe that it would be causing major gearbox problems. Since I do not have any problems shifting, unless I am causing them, this theory seems unlikely. A second theory centers around the car's connecting rods. The two options here are either a bent rod from overuse, or an out-of-balance rod from the factory's forging process. Again, this solution doesn't work because the noise would become more pronounced as rpm increase, and some vibration issues would be expected. The third and final theory centers on the engine's direct fuel injection system. To a much greater extent than a port or throttle body injection system, direct injection systems can vary the air to fuel ratio within the combustion chamber. At high load conditions, like passing a string of SUV's with New Jersey license plates, simply to aggravate them, the mixture is very rich. This provides plenty of fuel to make power and also cools the combustion charge, allowing for a higher compression ratio in the case of a turbocharged engine. At very low loads, like idle, the mixture can run at a very lean ratio, saving fuel. I think my engine noise may just be some occasional knock from this super-lean condition. Of course, there could be a screwdriver from JiffyLube running around the engine bay, as well.