I posted a similar topic a few months ago *Lou* - have bumped it for you.
Jimmy still rocks himself to sleep, and we've been quite successful in preventing the associated sound effects by popping his dummy back in his mouth if he throws it out of the cot before going to sleep. So most of the time he just rocks now, instead of making noise too. But in saying that, I want the dummy to go by the time he's 2, so I see your dillema!
As you say, it seems much worse to disturb them as it means they simply have to start the *going to sleep* process all over again, and therefore you have to listen to the noise for even longer .
Can you wear ear plugs while you're driving? I guess the only other thing you could do is try to plan the driving time around when DS is usually awake (not sure how many hours you have to travel though - if all day that may not work)?