Aerodynamic noise...
Further to the above, an anecdotal observation:
When I first flew the (very quiet) A330 I was amazed to discover that we could hear opposite direction aircraft pass 1000' above or below our altitude in exactly the whooshing sound that you'd imagine a cruising airliner would make. That's true of other 330s, 777s and the ever less common 747s. But its very subtle on the 787s that we (horizontally,at least) joust with, to the point that its perhaps more imagined than real. I have no trouble at all believing that a 787s passage is profoundly and audibly less traumatic to the air, and hence less wasteful of energy.