It is the sound of a peacocks call, in this case, the ones on Marguerite's estate. Peacocks have always tended to be the birds of favour for European Grand houses and in the example of the film, I think they are a metaphor of Marguerite's own personality. They make a terrible sound and yet they remain graceful and beautiful. Marguerite herself comments on how the female of the species attracts the male Peacock 'She pecks at his eye' which is an echo of her husbands mistress comment that the only reason Marguerite persists in her singing is to get her husband to look at her. In addition, she dons a peacocks tail-feather as part of her costume - if one thinks a little deeper about some of the themes of the film, such as voyeurism, the peacocks plumage is comprised of thousands of eyes and the last shot of the film is of a prying human eye looking down through the lens of a camera - us I suppose as the audience spying on the heroine but ultimately, completely on her side.
Thank you for your explanation, it makes perfect sense. I think I would have thought of Peacocks as well, hadn't I watched the film on an airplane with miserable sound quality and terrible headphones. It sounded like nothing but a cats meow.