If it's a CRT, which I assume it is, how long has it been in use? The last one I owned made a high pitched squealing noise about 3 months before it blew.

Are you using a typical PC speaker setup, which is magnetically shielded? Or regular home audio style speakers?

Try separating the monitor from the speakers and modem, power off & back on. If the issue stops, you've got your answer.

You could also try a minimal adjustment to the refresh rate or screen resolution.