You're asking for a vowel? No can do, no Egyptian script writes vowels. Moreover you're asking for a vowel-single consonant syllable, which Egyptian would simply write as a uniliteral consonant. Biliteral signs are only used when there are two consonants, what you're asking for would simply be written with the r mouth and nothing else. You may think, for example, that the uniliteral aleph vulture or w quail chick are vowels, but they are not. We pronounce them today as if they were vowels, but there is considerable debate about the "vocalic" consonants of Middle Egyptian. The double reed yod, for example, is totally unknown to us in Old Egyptian and then in Middle Egyptian it seems to be largely indistinguishable from single reed yod, a symbol whose precise value we do not understand! If the theory that initial yod stood not for a consonant but for an initial vowel (since in Egyptian uniliteral consonants standing by themselves generally are followed by an unwritten vowel, not preceded by one) then conceivably what you're asking for might be written with the yod reed followed by the r mouth. But that's highly uncertain, and in general back-construction of words is difficult in Egyptian, and nearly impossible with onomatopoeia