<< Find the size (most likely 60mm) and replace them with Panflo fans. They tend to be the quitest on the market. >>
I don't think this is a good idea. The Panaflo 60mm just doesn't move enough air. For instance, the Panaflo 60mm is rated at 12 CPM and 24dB. The VolcanoII, regarded as a quiet HSF, is rated at 36CFM (prob. 26) and 31.5dB. So the Panaflo can only move 1/3 to 1/2 the air at 3/4 the noise.