The Specialized hub that came on my Roubaix is almost silent, the Ksyrium Elite hum is pretty ratchety.
Why are some louder than others? It may be that changing teeth profile/springs to be quieter = less reliable, through softer materials or design. Also if hubs are very lightweight there is probably less material between it the pawls and outside air. Sound dampening measures can add weight too.
But i've learned to consider it a HTFU alarm too
