The squel is from the "chokes" (electrical devices that look like transformers with values in milli henrys). There was a option for chokes of half the value that made half the noise. These were designed for schools but none were purchased as schools could not afford it.
Try the head in neutral and see if the noise is still there. If not the it could be the "clutches", spindle or spline.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)