iMac hard drive noise?

Well i've never noticed this before but lately I can hear my iMac's HDD "spinning up". I've honestly never heard this before. I think it happens when I leave my computer for a while and I come back and click on something. I hear this "spin up".

I'm sure its nothing but call me paranoid because I swear I've never heard it before (or not!?)

Sounds like the settings in the energy saving preferences are causing the hard drive to sleep whenever possible. You can turn the option off.