I understand -- if it's not something you've heard in an engine before, it can certainly be alarming. Especially when it's a brand new car, I think we're all a little paranoid about anything that seems amiss (I know I am -- for the first week of owning my SE I thought there was something wrong with the suspension while turning, but I've concluded it's just the driving characteristics of the lousy Dunlop tires).
I'll admit that the first time I heard the injectors clacking I was wondering what it was about. After I saw that mechanical review video I posted in the other thread, I stopped worrying about it. It's an unusual sound that I wish Toyota had figured out how to engineer out of the system, but I think it's probably normal for this engine.
To further set your mind at ease, check out this video of a Camry making the same sound (it has a slightly larger version of the same engine as our Corolla):
If you read the comment section under that video, and browse message boards where people have noticed this sound in the newer-gen Camry, you'll see lots of people chiming in that it's a normal sound for Toyota's high pressure injectors. I'd guess that on startup/open loop the ECU probably does some cleaning cycle or similar on those injectors, which results in the louder-than-usual sound. Maybe the cycle runs longer in colder environments? You'd probably never have noticed it if you had the XSE with the sound-dampening material installed on the underside of the hood.