"Car horn" is "клаксон" or "гудок".
"To honk" is "сигналить" or "гудеть".
Also, children teaching includes "cats say 'мяу' (meow), cows say 'му' (moo), .... cars say 'би би' (bee bee)" (it really sounds as english word 'bee'). It is rarely used by adults, but 'бибикать' is something like children word for 'to honk car horn'.
By the way, foxes say 'тяф тяф'.