I have tried to coin a word for this tunefull condition as have friends, but we haven't come up with a winner yet. One reject is tuneration (tune plus part of perseveration). I work on houses and will find myself in the middle of the day humming "Here Comes Santa Claus" and this can be in July. I am sure there is a medical diagnosis if the condition becomes severe enough. The most common form is when one finds oneself humming the last song heard on the car radio on the way to work.
I think there may be a French word for this comdition.