Years ago while in college I attended a class on movie sybolism. One of the recurring items was the use of flies or the sound of flies to represent death. You hear or see flies…someone is going to die shortly in the film/movie.
Where did this come from?
Is there some book or reference that filmakers use to find these symbolic icons?
What other type of symbols exist like this?
I think your teacher is confused. The flies come after the dude gets whacked so they can eat the yummy body.
I doubt there’s a single director out there who works from a list of established symbols. I’m sure flies have been used to presage death, but so have oranges. Half of these symbols are probably unintentional on the director’s part anyway.
I think the OP is valid, to an extent. There is a cliche that exists where the buzzing of flies is present (how else could it be spoofed on the Simpsons? 
), and tracing the history of this would be informative.
However, if it was introduced as simplistically as the OP describes, then yes, it was a bad teacher (or they had a bad day 
)
I can’t quote it, but there’s a poem by Emily Dickenson that includes the sound of a fly in the window as someone dies. And as friedo says, they forshadow the decay of the body after death. A fly in the window is also often trapped and doomed to die there and clutter the window sill. So it’s not surprising to see the image cropping up.
I remember hearing somewhere that a rotating fan represents something - maybe death?
It represents being in a sci fi movie. Either that or grinding oppression due to corruption. Or heat. It could be hot.
Depends on how fast the fan blades are moving.
Abe Simpson: At my age, death is around every corner. [points and screams] Ahhh, death!
Lisa: Grampa, that’s just Maggie.
Grampa: Oh yeah. So I … ahhh, death!
Lisa: That’s the cat.
Grampa: Ahhh, death!
Lisa: That’s Maggie again.
Examples of movie shorthand for death are the beeeeeeeep of hospital equipment, and the bernnnnnn of a stuck car horn. Inevitably, someone has just died.