The noise that some dial up 33/56k modems make is awful, and the volume slider in modem properties never seems to stop it. Here's how I stopped mine. Make sure you're modem is NOT connected to the internet at the time.

Go to Control Panel/Phone and Modem Options and click on the Modem tab, click on the modem listed and click on properties. In the new dialogue box that comes up click on Advanced. In the text box under the words Extra Initialization Commands type ATM0 (number 0 not letter O). Click on OK, then OK again, etc. THe modem model I have is Conexxant HSF v.92 56k etc. But it should be safe to try on other models.