What does "mu" mean as an interjection?


I'm reading the _Dragon Ball_ manga and characters keep sort of stammering
"mu-mu-mu" at various moments, for example below. What does this mean?

[![enter image description
here](https://i.stack.imgur.com/JCJsR.png)](https://i.stack.imgur.com/JCJsR.png)




I love this comic too. Mu mu mu expresses frustration in a calm way.




I love this comic too. Mu mu mu expresses frustration in a calm way.

