I need some help

[Nismo 240]

Heres the deal i dont know much about turbos this is my first one i have a 1989 240sx with a redtop sr 20 when i start to reach boost starting at about 3lbs and higher it makes a high pitch wine can anyone tell me what it is.Here is a video if it will help you can here it before the compressor surge.
100_1273.flv video by bignismo240 - Photobucket (http://s259.photobucket.com/albums/hh318/bignismo240/?action=view&current=100_1273.flv)