A lot has been written about the performance advantage of adding a memory card or usb stick to your Vista machine to make use of the ReadyBoost feature but one interesting side effect is that it can also reduce the amount of access needed to your hard disk and appear to make it quieter.
To quote the official Microsoft blurb:
The flash memory device serves as an additional memory cache—that is, memory that the computer can access much more quickly than it can access data on the hard drive. Windows ReadyBoost relies on the intelligent memory management of Windows SuperFetch and can significantly improve system responsiveness.
Now a lot of this data would normally be held on the hard disk and will need reading regularly, this can be the cause of the chuntering you can hear from lots of hard drives. Putting in a high-speed flash device can significantly reduce this. Obviously its not going to make the drive any quieter when it actually has to access data but it certainly has had avery beneficial effect where I have tried it.
Performance and a quieter life - two for the price of one :-)
Edit: I should point out that the cache is still written to disk so if you swap programs a lot it will still need to write out the changes and you will see less benefit in terms of noise. Q: Why does it still write to disk? A: Have you never pulled out a USB key by accident? The system need something to fall back on.