performance - Laptop's hard drive doesn't really spin down

I have a new laptop with dual boot to both windows 7 and ubuntu 11.04. While in ubuntu, the laptop gets significantly warmer than in windows.
My suspect is the hard drive, because I feel it's especially warm above it, and also I believe I feel its vibrations constantly, even when I'm doing nothing with the hard drive and its led is off. (BTW, I checked, and "spin down hard drives when possible" option is checked.)

Whenever I try: hdparm -C /dev/sda
I get: drive state is: active/idle

And the interesting thing is, when I tell it to sleep: hdparm -Y /dev/sda
It says: issuing sleep command
And I can actually feel it stops spinning, but only for a couple of seconds. (It goes back spinning without me triggering anything)

My question is, what can I do to make my hard drive calmer?

(It's a SATA 500GB 7200RPM)