I occasionally get that same rattle on startup. It's not consistent...which makes it even more annoying. It might happen when the engine is cold...but it might happen when it's warm, too. I've probably only heard it once this week. Like Mark said...I kinda just chalk it up as a quirk at this point.
I have also noticed (what I think is) piston slap. This time of year when the car is cold, I get a very light knocking noise out of the engine until it is nice and toasty. Based on some of the teardown pics that Fummins has posted, I assume it's piston slap.
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View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)