Need help with CNC Gildemeister CTX 400E (year 1994), the main spindle motor is making a horrible noise, so we change the 3 main bearings inside the motor, but we still getting the same noise special when the spindle runs in slow speed 300rpm, we think it’s the main spindle drive, can anyone tell me what that problem and how I can solve it (I tested the motor in a different machine runs good without no noise)

Thanks