Who Put the M before "Bye"?

There is no m in bye, and yet half the time I end a phone call, I hear the person on the other side of the line say, "Mmmbye." It's usually women who do it, and from what I can tell, it is those who were born or grew up here. What's with the m? Is it because they press their lips together in a smile before they say bye? Or is it like a regional accent the way Bostonians add Rs where there are none (idea - idears), to taking away Rs (park - pahk, yard - yahd) where there are? Whatever it is, it never fails to make me smile. It makes me wonder who started it and how it spread to a whole segment of the population. Any idears?