These are the combination of sounds in a word that imitates or suggests what the word refers to. For example:
hiss - for the sound made by a snake
cuckoo - for the sound the cuckoo bird makes
sizzle - the sound you hear when bacon is frying in the pan
Can you add to the list?
I love this one in tagalog (filipino) : nangangagat: bite
Tick tock!
Drip drop!
...and "whinney" and "neigh" for the sound a horse makes.
"Smack" for the sound of a slap.
I heard that many of the english verb come from onomatopeias, like to scratch, to sneeze, to whip, to hit....
Quack!