"What does the fox say?" Onomatopoeia, and Alien Languages claims there's no onomatopoeia for foxes:
But you don't find fox onomatopoeia in this context. Foxes tend to do
one of two things: either they are silent, or they speak like humans
do. It's certainly a testament to the fox's slyness that it's
attributed with human speech, which fits quite well with its trickster
qualities (and of course there are many myths that have the fox
transforming itself into human shape, too).
The article goes on to cite a Wired article claiming that some of the sounds made in the Ylvis song The Fox (What Does the Fox Say?), such as "Chacha-chacha-chacha-chow", are accurate, but it doesn't indicate that they are commonly used onomatopoeia.
Are there any commonly used onomatopoeia for foxes in English?