According to the website of Sciences & Avenir (September 2015):

Recent studies have shown that giraffes of the species Giraffa Camelopardalis evolve in a somewhat original structured social system. Like hyena, elephant and African buffalo societies, giraffe societies require a sophisticated voice communication system to facilitate these social dynamics. But then, how do giraffes communicate? Until now, it has been impossible to know what kind of sound they emit. Scientists even once thought it was impossible for these mammals to make sounds. Indeed, despite their developed larynx and laryngeal nerves, it is difficult for them to generate enough air through their long necks to produce vocalizations. It was later discovered that they emit infrasound, sound waves that are below the limit of human hearing. Today, a new discovery: according to a study published in the journal BioMed Central (BMC), giraffes hum!

This was proven by the analysis of 947 hours of recordings in three European zoos: Berlin Zoo, Copenhagen Zoo and Vienna Zoo. For 7 months the giraffes were recorded day and night. And it seems that the darkness is conducive to chatting. A total of 65 humming sounds were collected: 34 in Berlin Zoo, 9 in Vienna Zoo and 22 in Copenhagen Zoo. Humming is a low-frequency sound, around 92 Hz, with a structure that includes a wide variety of notes. Unfortunately, researchers have not been able to discover whether these sounds are used to communicate with each other or to “introduce” themselves. It is indeed known that in other species with similar social structures, vocalizations are used to deliver information about the age, gender, reproductive state or sexual arousal of the transmitter.