Free hand turn the spindle with the power off. If you still hear the noise... then it's most likely mechanical i.e. bearing, Spindle belt ect.. Noise only with the power on is generally electrical or electrical mechanical. i.e loose encoder, wires, or board. Pegging the load meter...maybe a motor driver problem or connection to same??
Any error messages? Burned insulation smells? Other thoughts, shift idler bearings... How old is the machine?