After a while he fancied he would like to go faster, so he began to cluck with his tongue and cry "C'ck! c'ck!" The horse broke into a smart trot, and before Hans was aware he was thrown off—splash!—into a ditch which divided the highway from the fields, and there he lay.

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(How could we pronounce/say the sound, "C'ck! c'ck!"?)

It's almost like "chick - chick" except it's done at the side of your mouth, and the vowel is not sounded. It's very hard to describe, and I can't find a pronunciation symbol to link to someone's demonstration!