mmmm...okay
1) A negative okay. 2) The person is considering your response or they are considering your response in a known believing way.
David: I think I hurt his feelings. Here’s a tissue for his issues.
Amanda: Mmmm...okay.
Adam: I didn’t do it. I was at home the whole time.
Sarah: Mmmmm...okay.
Amanda: Mmmm...okay.
Adam: I didn’t do it. I was at home the whole time.
Sarah: Mmmmm...okay.