iMac Whirring Grinding Noise

Hello,
Today I noticed that my iMac was making a whirring and almost grinding noise. Everything works fine and the noise seems to only occur at specific times. One time was when iTunes was accessing a CD of spooky sounds I just placed in the drive. No, this grinding noise was not one of the tracks on the CD! The sound also occurred earlier, but I can't remember when it was doing it. Any suggestions other than making sure all my info it backed up? What could be causing this?
Ginny
PS: I know Macs frequently make a whirring noise, but the deep throated grinding is definitely new. I am a longtime Mac user

Only happens when you have a CD in the drive?

Yes good question, is it for sure only happening with a CD/DVD in the drive? My iMac's Optical drive with some media makes a very deep throated noise but with most other media it's very quiet.