if a woman ever says "mmmm-hmmmm..." instead of "you're welcome" after I say "thank you", I punch them in the back of the face. I have punched several women in the backs of faces because of this.
when someone says "thank you", you say "you're welcome", or "no problem", or even "don't mention it". You never grunt and make a noise like "mmm-hmmm..." and walk the F away. It is better to say nothing at all than to do that. I will punch you if you say that to me. Women do this A LOT. Waitresses in particular. But WOMEN in general I've found are the main offenders.
I'll kick you in the neck next time you say "mm-hmm..." and then turn around and walk away. I'll kick you in the back of your neck.