If you smell burning computer parts cut it off and unplug it.
I doubt that is your problem. If you can not use your modem how did you get here to post? (This is not a joke. I am trying to figure out how bad the problem is.)
Does it happen all the time or just every now amd then?
Do you feel OK about opening up your computer and working on it? Have you ever done that before? If no do you have a friend that does and can help?
If you feel OK about it remove or unplug everything from your computer but the motherboard, ram and cpu. Power it up. Does it still make the sound? If yes post back here.
If no. Power down add in the hard drive. Power up. Does it still make the sound? If yes post back here.
If no. Power down add in the modem. Power up. Does it still make the sound? Can you dial out? If yes post back here.
Try adding in each other peice until the sound comes back. If you feel OK with it open your computer case and power up and see if you can track down where the sound is coming from. Again if you have not done this before and do not feel OK with it do not do this. You need to know how to ground yourself and how to hanlde the computer parts. It is very easy but until you know how you might damage a part. Even if you know what you are doing from time to time you can break something.
Granamere