Active hard drive silencers are just an enclosure that goes inside your case. They add another component that might fail, however (they have fans). You could also use external esata enclosures like the Antec MX-1.
You can suspend them if they are transmitting sound through vibrations into the case (google hard drive suspension). You could also dismount them and let them sit on something that absorbs vibration, like a block of foam. Be aware though, some heat is removed from the drives through the case, and they tend to get hotter when suspeded or sitting since they are not in metal-to-metal contact with the case.