I am going to name this condition "I'll Hum In Error A Show Tune"
I have it narrowed down to one in about 3,247 movies.
(Pause as I dig through my DVD's for the answer after being unable to find it on iTunes)
Its from the 1981 movie Chariots of Fire, chapter 19 on my DVD. Its the alternating merged practice scenes where it switches back and forth rapidly between Eric and Harold working with their trainers, and then the music ends just as Harold runs through the finish line and Mr Sam Mussabini clicks his stopwatch.
(An hour later) I actuaLLy figured it out several minutes ago, but then I had to spend time finding a pair of double A batteries for the Blu Ray player remote control device, to confirm it and figure out eXactly how far into the movie the scene was located.
I probably should not have watched this movie 20+ times. I should have stopped at 14.