The SilentBoost is NOT 21dBA. The Panaflo L1A is rated 21dBA and spins at 1700RPM. The Panaflo that the SilentBoost uses is an M1A and spins at ~2450RPM. Unless those slits on the fan that the SilentBoost has signficantly decreases noise levels, I always call BS on that 21dBA rating. Furthermore, there are 2 models of the SilentBoost. One comes with Panaflo fan and the other comes with a regular ball bearing fan. The model number is slightly different, but they're both SilentBoost.
Oh yeah, I used to have the SilentBoost myself and have installed it on other people's PCs 🙂