Are all single-note whistles "hooters"...?
No.
The nickname "hooter" comes from the sound made by certain whistles.
From a distance, a deeper toned "hooter" whistle sounds like an Owl "hooting"... "who" "who". Find some audio from N&W 1218 excursions in the late 80's.
The whistle on 463 is a bit higher note, but the same idea.
Reading's freight whistle is an even higher note, and sometimes referred to as a "hooter".
A Pennsy Banshee sounds much more like a woman screeming. (My appologies to the SPF's).
Hope this helps.