Neoseeker published a mini shootout of 3 different hard drive cooling /silencing products.

When it comes to the three products' silencing abilities, not much can be said for the Xilencer. Rather than silencing the drive, it actually amplified its vibrational and operating noise, just as I had suspected. The two large metal covers, making direct contact with the drive, act as amplifiers. Padding is a must if there is to be a reduction in noise for such a small enclosure. The SilenX Luxurae and Cooler Master CoolDrive, on the other hand, do what they are intended to do. When placed in either enclosure, the WD Raptor was noticeably less audible. Any audible noise I may have heard was during hardcore thrashing, and even then it was very muffled. On the other hand, when placed in the A.C. Ryan Xilencer, the WD Raptor's thrashing sounded more metallic than usual, undoubtedly due to the lack of padding as previously discussed.

Neoseeker 3 way HD Cooler/Silencer shootout