The Usenet Oracle has pondered your question deeply.
Your question was:
> Oracle! Oracle! Thou art magnificent!
> Grant thy humble slave the benefit of thy wisdom!
>
> Why is it that some floppy disks make a shhukka-shukka-shukka sound
> when placed in a drive, and some do not?
And in response, thus spake the Oracle:
} According to the Grand Unified Breakfast Cereal Theory of the Universe,
} the molecular structure of everything (including floppy disks) is
} modelled on American commercial breakfast cereals. Let's take a few
} floppies at random and see how they sound:
}
}         Sound                              Description
}         =====                              ===========
}
} "Snap! Crackle! Pop!"           Often indicative of a defective disk,
}                                 this sound is characteristic of
}                                 diskettes composed from a rice paper
}                                 substrate [try eating the diskette
}                                 envelope (5.25") or "Plastique [tm]"
}                                 wrapper (3.5") to make sure.] As it 
is
}                                 edible, this diskette type is
}                                 particularly useful in data-security
}                                 applications. The Japanese are also
}                                 working on a soybean-based version (the
}                                 miso flavour has been particularly
}                                 popular in tests).
}
} "ChockaChockaChocka!"           Chocolate-covered diskette; this was
}                                 once used extensively on Apple II
}                                 computers. As they used to say, "an
}                                 Apple a day keeps the disk doctor
}                                 away". (This diskette type was
}                                 abandoned when the Macintosh went into
}                                 production, a move which gave many the
}                                 pip.)
}
} "FRRROOOOOOOT!...LOOOOOOPS!"    This onomatopoeically-named diskette
}                                 is not recommended unless you also use
}                                 a spell-checker.
}
} "WeeeEEEEEEEEEeeeeEEEET!"       Shredded Wheat [tm] inspired design,
}                                 once a favourite with operating system
}                                 programmers for archiving their
}                                 multi-tasking kernel software. Now used
}                                 for sensitive data that is to be
}                                 destroyed afterwards (saves the user
}                                 time as data on the disk is already
}                                 shredded, and in byte-sized chunks!)
}
} "MmmmmMMMYYYOOOOOOZZZLLEEEE"    Floppy disk structured after wholesome
}                                 natural meusli. This design ensures a
}                                 good rapid movement of data to and from
}                                 your system, particularly good if your
}                                 data contains 0xBC (B-12) characters.
}
} "BrrrRrrRRrRAAAAANNNNNNNN..."   The boring counterpart of the meusli
}                                 disk, this diskette will get the job
}                                 done, at the expense of being dull and
}                                 dreary. The diskette of choice at IBM.
}
} "Shhukka-shukka-shukka!"        Generic sugar- (shhukka-) coated
}                                 diskette. After a few years using these
}                                 diskettes, your disk drive's read/write
}                                 circuitry will rot. Avoid at all costs!
}                                 If you must use them, make sure you
}                                 clean your drive head immediately
}                                 afterwards.
}
} (Note that Kellog's has recently withdrawn from the diskette market,
} as their corn-based products were found to be a bit flakey...)
}
} You owe the Oracle a cup of coffee, orange juice, and some toast.