cookybiscuit :
Just trying to drown out the noise.
If you have a good case, it'll feature thick steel panels and silicon grommets for the HDD mounting so that the vibration and noise isn't transmitted through the chassis of the case AND whatever noise that there is in the case decays rapidly because there are no resonances.
If you have to add damping/acoustic absorption materials to the case, I would strongly discourage applying it to individual components. Just about every PC component I know requires a free-flow environment to passively cool the critical components. HDDs are particularly sensitive to temperatures. Add the acoustic treatment to the case panels instead. Heavy case damping materials are meant to cure vibration, rattle and low frequency issues, while acoustic absorption foams are meant to do the same for higher frequencies.
HOWEVER, good acoustic absorption materials are expensive, and can cost nearly as much as a good quiet case (like the Antec Solo series, or the Fractal Design quiet cases). Recently the Solo I (an excellently quiet full ATX case, see www.silentpcreview.com) was being cleared out everywhere and the going prices were about 50 bucks. Most packs of acoustic treatment for PCs (good ones that actually works, not just a thin light foam) costs more than this.