Shrieking noise, Nazgûl or his beast?

Question

Who makes that shrieking noise, the Nazgûl or the beasts they ride on?

Answer 1

Book 1, ch.4:

"A long-drawn wail came down the wind, like the cry of some evil and lonely crea­ture. It rose and fell, and ended on a high pierc­ing note. Even as they sat and stood, as if sud­denly frozen, it was an­swered by an­other cry, fainter and fur­ther off, but no less chill­ing to the blood."

Book 1, ch.12:

"...and from the Rid­ers came a ter­ri­ble cry, such as Frodo had heard fill­ing the woods with hor­ror in the East­farthing far away."

Book 2, ch.1:

"‘Be­cause they are real horses; just as the black robes are real robes that they wear to give shape to their noth­ing­ness when they have deal­ings with the liv­ing.’

‘Then why do these black horses en­dure such rid­ers? All other an­i­mals are ter­ri­fied when they draw near, even the elf-horse of Glo­rfindel..."

‘Be­cause these horses are born and bred to the ser­vice of the Dark Lord in Mor­dor. Not all his ser­vants and chat­tels are wraiths!"

It's the call of the Nazgûl themselves. They were riding normal horses up until they lost them at the Ford of Bru­inen. The flying... things... don't show up 'til much later in the story.

Answer 2

Short answer: It must be the Nazgul, not the fell beasts, because they hear the sound in the Shire, when the Nazgul just have horses.