My 13mth old does this every now and again and has been doing it for awhile.   I find that if he is bored then his starts up.   He does it alot in the supermarket.  LOL

Anyway it is just him finding his voice.  Try ignoring it and not running to him everytime he does it or it will become an attention seeking thing.   Try and teach him some words and other sounds.  

Has he been doing it for a long time?  If not, it may be that he has just 'found his voice'.  If you actively discourage it and get everyone within earshot to do so, (strangers or not), then it will eventually fade out like most bad habits. 

Like most behaviours/conditions, if it doesn't seem to improve over a few days, then seek medical advice, just in case it is a sign that something else is wrong.