The readings are both kun'yomi vocab readings, probably because a 鼻 is a body part, and body parts do that to jukugo words. You know 鼻's reading from the kanji (はな) and you know 歌's reading from the vocabulary word 歌 (うた). Combine them together and you have a very nice humming tune and reading, はなうた.
Koichi was humming while making his nose swell.
Viet was happily riding his bicycle while humming.
LMAO! You are just terrible at humming.