Why are higher-end freewheels so much louder when coasting?
Most of the noise comes from pawls on the freewheel hitting against the splines on the engagment surfaces which makes up the racheting unit.

Some reasons for the noise between freewheels?
Tension on pawls could be higher causing more noise as they glide over the engagment surfaces

High end freewheels have more pawls and engagement points than lower end freewheels, so there are more ridges in the engagement surface and more pawls hitting the splines on the engagement surface. The reason this is desirable is that more pawls and engagement points means faster engagement when you start pedaling.

Different grease (or less grease) could also be used inside higher end freewheels that is less viscous and provides less resistance, allowing the spring action of the pawls to cause more noise as they float over the the splines on the engagement surface since they are less restricted by the grease.

Of course there are exceptions to these.  Some hubs don't use your standard racheting mechanism and use a 'roller clutch' instead. The roller clutches tend to be very quiet, but are more prone to failure.Here's a good description of how those work.  http://pardo.net/bike/pic/mobi/d.winners-hub/index.html