My DD (who is 9) did this for a period of about 2 months last year. Her tic was more of a quick "hum"...she was completely unaware of it until I pointed it out. I then felt bad because I didn't know it was a tic before I asked her to stop doing it . At that point, she became sensitive to it and I think the vocalizations actually increased! I had read that sometimes these tics can be due to medication, but she had been on the same medication and dosage prior to the tics. My DH asked me to stop talking to her about it, as it was causing her undue stress (I think part of it is I don't do well with noises).
In any case, it disappeared about as quickly as it came. No rhyme or reason...I suspect it may be a developmental thing too. I would suggest, unless it's truly bothering him (it sounds like it may irritate his throat after awhile), not saying anything. It will probably go away on it's own. In our case, discussing my DD's tic made her more anxious and exacerbated the noise....