Children don't get much more silent than when they're asleep (and any noises they do make tend to be cute ones). Getting your kid to sleep on the plane or in the car isn't the easiest thing to do, though.
If you're cool with letting a dose of Benadryl do the dirty work, which some perfectly good parents are, you're going to have a much easier time of it. Otherwise, you'll need to make some scheduling adjustments so that nap time falls during travel time.
If your little traveler is an infant, that could just mean shifting naps up or back an hour the day you're travelling. With toddlers or older children, you don't have as many naps to work with, so you could be looking at planning your travel time around nap time. You'll likely find that a slightly higher ticket price or catching the end of rush hour is worth it if it means you'll have a sleeping angel in your lap or the back seat for a chunk of the trip.
Up next: Objects in motion tend to wear themselves out.