Here's another person who has reported hearing it. With a better description too (robot dying).
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forums.macrumors.com/sho ··· =1279821Curiously there seem to be some who have it and others that don't. I have owned in the past a 3G and a 4 and neither of them did this.
Here's someone else questioning why his phone doesn't do it but his girlfriend's does:
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apple.stackexchange.com/ ··· ing-toneInteresting.