Bringing his host down and kneeling he heard twine with his second bell the first bell in the transept (he is lifting his) and, rising, heard (now I am lifting) their two bells (he is kneeling) twang in diphthong.
There are no more uses of "diphthong" in the book.
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a falling diphthong
Now his voice abruptly lost the faint throaty Negroid quality with which it had been touched; in moist metamorphosis the Southern accent faded and died, replaced by thorny Polish diphthongs that were in almost exact mimicry of Sophie’s own speech.