Scariest sound I ever heard was a fox "screaming"! I was about 12 and out in the fields of my Grandfather's farm just before dark.
I heard what sounded like a women getting murdered, x10!, in the woods behind me.
It was about a 12 acre field and a fox came out on the opposite side of the field from me, stood there and did it again.
My uncle said he was also scared by them when he was a kid.