The word "heehawing" refers to the braying noise made by a donkey. Synonyms for this word could include "braying," "neighing," "bellowing," "bleating," or "yowling." These words can be used to describe other animal sounds as well, such as the "neighing" of a horse or the "bellowing" of a cow. Using synonyms such as these can help to add variety and interest to writing, avoiding repetition and providing a more nuanced description of sounds in the natural world.