Is it that the drive sounds like it's constantly reading / writing data, when the system is idle? (like the common 'buzzing' sound you hear?). The platters in hard drives always spin when connected to a PC that's on. I thought platters always spin at a constant rate
If you've got anti-virus software, check to see whether it scans in the background when the system is idle. It may be causing the activity.
Also, check to see if the indexing service
is enabled in Windows. It's supposed to speed up the process of searching files on your hard drive, but for most it's not important, and is known to use up a fair bit of CPU cycles. The indexing service is designed to perform intensive hard-disk indexing operations when the computer is idle. This might explain why there's HDD activity when you're not using it.
Read the following guide to disable the indexing service:
I disable it on all my PC's.
However, other processes running in the background may be causing HDD activity when idle.