We had exactly the same problem with DSS1 (16) until about a year ago. I kept reading books about how teenagers were supposed to be silent and moody, and DSS1 never shut up and talked about nothing at all for hours on end. Car journeys were HELL.
I don't actually think it's normal; but I think it isn't infrequent.
DSS1 is not über-mature [massive understatement emoticon]; your DS is only 12 - I'm sure it'll sort itself out. But be tough - if he is annoying you, tell him and sanction him. It is good for boys' development to have the rules of society rammed into them!
There's a thread in the special needs board today about verbal stimming maybe read that see if it helps at all, sorry no other advice except this is what it could be. My dd is 7 and makes odd noises she has ADHD and we were told it's verbal stimming
My ds (11) does that but he has asperger's so for him it might be related to that, it could just be normal for some children but if you are worried then talk to his teacher or your GP, even if it is just to put your mind to rest.