When I don't know how to reply in a conversation or text, I need a filler word to allow time to think of an answer so I say Hmmm. Hmmm is my thinking sound!
DH: Baby, what do you want to do this weekend?
SS: (5 minutes later) Hmmm
HMMM is the term used for approval during the fingering of an anal passage with a girl. The term "HMMM" is also followed by a smiley face. The "HMMM" is also done with a low frequency such as a black man enjoying some chicken, "MMM M MMMMMM" but with a "HMMM" and a smiley face.