literally its "byuru byu"
I kind of agree that it is that onomatopoeia,
but man, that is a really weird writing of yu (ユ) on the first "Byu",
I can see it on the second one, but the first one?
Even if it is somehow using hiragana, I can see the first stroke of ゆ, but the second stroke is 90° off.
That writing is way closer to either タ or 々, but the way the strokes crossed each other is also wrong
and it is not ヌ either