Could also be
a rattling HD, indicating eminent failure. Be sure to backup up frequently Until you know if this is HD or Speakers.
If changing the Audio Output or Volume does not change or stop the noise, boot from a startup DVD/CD or external drive, then use Tools or Utilities->Disk Utility->Disk First Aid:Verify Disk to check for problems.
Run the machine for 4 or 5 hrs. If the behavior continues you'll have to open the machine to determine if it's a fan a bearing noise, or if it's coming from the HD. You may have to employ a probe (a dowel or screw driver) one end lightly resting on device other end to your ear, to isolate the noise.
Remember to return and mark accepted the answer that best solves your problem.