My daughter asked me this question when she was about 10 years old. But I couldn't tell her the answer then. That was many years ago.
Perhaps there is no word to describe the sounds made by a giraffe. I wish I knew someone who is a zoologist with an MA in English, as Marius suggested.
You know, I have no idea what a giraffe sounds like, but the day I was at the zoo and heard a hippo make its "hippo noise" I was astonished! It sounded like a very bad, very loud trombone player. Until then, I would have thought they just grunted or something.
From the Oakland (CA) Zoo website: 'There are a number of misconceptions about giraffe. One is that they cannot make any sound. Although generally quiet, they have been heard to grunt, snort and bleat.'