Whistling noise on earpiece

this can be caused by a few things, depending on the type of phone and where it is....if the phone has a built in amplifier, the whistling could be caused by acoustic feedback due to the volume level being set too high, also your phone could be picking up some stray frequencies from somewhere, eg flourescent lights, or radio frequencies, a hearing aid, or there could even be fault on the phone line.... sorry to seem so vague, but without more specific information were fishing for clues.