At last, a long time question of mine has been answered. Thank goodness for YouTube and Wikipedia.
In many, many films you can hear the Wilhelm Scream.
This is the Wiki description of it.
See
this clip for several examples. (***Warning - Clip compilation contains scenes from
Howard the Duck and the
Star Wars Holiday Special.***
)
I heard this scream so many times in movies that I just knew it had to be some inside joke or homage.