"Sensei," I asked after class. "If inu says 'wan-wan', and neko says 'nya-nya'...what does kitsune say?"

She seems excited that I'd be interested in Japanese animal noises. She eagerly replies "kon-kon!"

Watching lots of anime has confirmed that fox does say "kon-kon!" Therefore, we are to assume than "kon" is how foxes yaff in Japan.

The Kanji is "uso" which to my understanding is lie, or falsehood( Yan-yan, a Japanese snack informs us "fox-beware of lies") to which a fox deftly replies "Kon" in hiragana.