Yeah, it's definitely a vocal, maybe even an auditory stim. Maybe something he heard on his video game or somewhere else that he's imitating... My son does things like that all the time. He "vrooms" his cars, in other words, he makes car sounds all the time when he plays with his cars... which is constantly. We've just managed to teach him that it's OK to do it, but there are certain places that he has to be quiet or even VERY quiet when he does it. It's a slowish learning process, but with time, your son will get it eventually too.
Stimming is a good thing, it is very calming to autistics and aspies, it kind of keeps them "centered." As much as I get annoyed with it and many of my son's other stims or eccentricities (like jumping up and down on his bed or trampoline or spinning in circles until I feel like I'm
going to throw up), I have to just go with it, because I know it's what keeps him sane.
I've found I have quite a few of them myself... one is that I like to chew on things...straws, pens, etc. Another is that I have to always be moving. When I'm sitting, my legs are always bouncing... etc. You get the idea. I've learned over time when these things are acceptable and when they are not... with your help, your son will too.
Keep your head up.
and maybe buy yourself a pair of earplugs for those especially loud days.