A moment when you realise about something you haven't thought about so deeply.
That thing you said made me go hmm...
When you sneak up on someone, and goose them with your finger into their butt, and say to them “hmmm hmmm hmm”!
I had a dream you snuck up and goosed me! All the while, you were kept muttering “hmm hmm hmm”! It is a real enigma, like the great pyramids. It made me think, he got me the “hmm” pyramid.
HMM is used when being sarcastic or just thinking
HMM, yeah thanks captain obvious
HMM, yeah thats a good idea