What would cause a computer to make noises like dialing a fax machine?

Even with the computer turned off, it is still making this sound. About every four minutes it makes four tones like it is dialing a fax. I do not use a fax with this computer. Please help!!
I have Road Runner through Time Warner Cable. I am using Firefox 3.5.3.
The computer was the only thing besides my tv that would have made any noise. The beeping was definitely coming from the computer. There was no fan noise and no HD activity.

By: Jana N.

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