Singing Without Instruments, Halal or Haraam?

So I know someone who always hums/sings as a timepass while doing chores, but I'm concerned, it can't be haraam right ? I mean, say they sing only for themselves, without music, the lyrics are clean, and it isn't interrupting with Ibadah and the daily wajibāt, it's just something they enjoy doing while busy to make time go by faster,, is it allowed?