This should be an easy one but I'll be damned if I can find it!
It is a sci-fi horror flick like "Invasion of the Body Snatchers" but somewhat different. As I recall, the viewer never saw the actual process where humans were taken over by the alien presence. The human replacement had no emotion and when confronted with an actual human, it would let out a high-pitched squeal as a warning to the other aliens.
The movie follows two people who seem to be the last of the humans. They adopt an emotionless persona to hide among the now-alien population. They meet at the end of the movie, I believe in a park, where the woman smiles at the man thinking that he still is in hiding when the man lets out the high-pitched squeal of the alien! The movie ends there.
What is this movie?