What causes the noise that race wheels make when you are not pedaling?
Most bikes have 'freehubs', a design Shimano came up with that is brilliant because it places the wheel bearing outboard of the sprockets rather than inboard, as per the 'freewheel' design that preceded it. Inboard bearings place a shear force on the spindle when going over bumps that can break the spindle.

Anyway, Shimano patented the freehub, and rightly so. Their freehubs run reasonably quiet, (with a lower 'note' on the ones with cheap steel sprockets attached to them).

Other manufacturers are not able to licence the freehub design or are unwilling to pay whatever licence fees Shimano charge. As a result they have their own technical workarounds.

The pawls of a Shimano freehub are placed deep inside the freehub body, the noise they make when freewheeling is therefore insulated, i.e. quiet. Other makes place the pawls inside the hub (Campagnolo) or else they use ratchets instead of pawls inside the freehub body (DT/Swiss). Hence the different noise characteristics of these up-market, high-end freehubs.

Here is the freehub body from a typical, recent Zipp wheel. As you can see they have not used the original (and totally excellent) Shimano design, the pawls are placed in the wheel, not the freehub-body. Coupled with the aforementioned 'guitar case' of the disc wheel a loud racket is made.