Bought an iMac 27" with 1TB HDD memory.Have not noticed any noise from the HDD when I play it in the  day time and tonight I play it I heard some funny noise coming out from the HDD.The noise will become louder when i open some program such as iPhoto.

I wonder if any one here using an iMac 27'' having the same "problem" with the HDD when they turn on the iMac in a quiet environment?

P/S The sound is similar to the sound of "water boiling"