Is it usual in anime/manga/japanese to use ブツ as onomatopoeia of mumbling?
ブツブツ or ぶつぶつ (the same word, just in katakana vs hiragana) is a standard Japanese onomatopoeia for mumbling, muttering, or grumbling to oneself. This list of Japanese onomatopoeia includes ぶつぶつ in it, along with many other onomatopoeia that show up in manga.