For a buck’s worth of materials, you can quiet your PC by damping your hard disk’s vibration:
As the pictures show, the drive is essentially suspended on the stretched elastic. The resilience of the elastic stops all vibrations from passing from the drive to the case — or vice versa, for that matter.
When I showed one of my suspended drive systems to my favorite local dealer, it was the complete absence of vibration in the case that amazed them the most. They could not tell when the PC was turned on by the usual vibration of the case. They found it eerie.
Keep in mind that you’ll loose some of the conductive cooling that you get when the drive is mounted to the case, so it’d be smart to do this in cases where there is decent airflow or find a way to attach some sort of heatsink to the bungeed drive.