It may be the heads of the optical drive occasionally realligning themselves, or the hard drive heads doing that as Spotlight is always indexing in the background. Backup your data frequently as my FAQ* explains:
http://www.macmaps.com/backup.html
Listen to music to obscure the sound. If your machine starts exhibiting other problems, see if a simple erase and installed system will exhibit the same problems. If it does, you may have a hardware issue that AppleCare needs to look at, or it may be normal. If your machine is out of AppleCare, you might want to elect replacing the hard drive with a quieter SATA drive, and optical drive that is
http://www.patchburn.de/ compatible that is quieter as well.
- * Links to my pages may give me compensation.
