Alliteration - the commencement of two or more stressed syllables of a word group either with the same consonant sound or sound group (consonantal alliteration), as in from stem to stern, or with a vowel sound that may differ from syllable to syllable (vocalic alliteration), as in each to all. Compare consonance ( def. 4a ) .

Although, that's probably higher than diphthong already if I understand the rules correctly......