No, あのう has no kanji.
From Tofugu’s article on Japanese onomatopoeia:
Onomatopoeia are written using either hiragana or katakana. While there are no definitive rules saying when you should use one or the other, in Jazz Up Your Japanese with Onomatopoeia , the author states that hiragana is used for “soft sounds” and katakana is used for “hard sounds” and emphasis. You’ll see lots of back and forth in which one is used the more you read, which is just another reason why learning both hiragana and katakana is really important.
でしょう would also be another good example of something that has no kanji equivalent as well.