* Noun: A word that names a person, place, thing, or idea. Examples: dog, city, book, happiness.
* Noun Phrase: A group of words that functions as a noun. It usually includes a noun and any words that modify it.
In this case, "train whistle" is a noun phrase because it describes a specific type of whistle (the one on a train).
If you wanted to use a single noun, you could say:
* Sound: This is a noun referring to the noise itself.
* Whistle: This is a noun referring to the instrument that makes the sound.