Hi there:)!
We put "bubbles in our mouth". Hilarious in kinder because of course they have got big puffed up cheeks and way overdo it:P
So when it is a "moment of silence" moment (usually after we have done the TEach!Okay! and I am bringing them back) I usually say something like:
"Claaaaasssss"
They respond: "Yeeeeeessssss"
I say: Class! Class!
They say: Yes! Yes!
I'll say "HAnds up high"
They say: "Hands up high"
I say: Hands out wide!
They say: "Hands out wide"
I say: Bubbles in your mouth!
They say: Bubbles in your mouth!
And then I say "Hands and eyes" (which sounds like HMMM HMMM HMMM cuz they have a bubble in)
They repeat.
I have taught them about "quiet bubbles". We listen for the air conditioner. I tell them that when I can hear the air conditioner I know their bubbles are quiet.
Now if the humming is happening during their work time~ my suggestion, something that I do during all of our work times, is to play "brain" (classical) music.
Hope this helps!
PS: I went to a school site that saw me use the "bubble" and they changed it up to where the students would decide "what" they wanted to load their mouths up with (chocolate chip cookies, brownies, etc.). I've never used that modifications but the kiddos seemed to get a big kick out of it!!!!

Andrea