Loose bolts will allow the reducer to move or deflect about the foundation and result in noise. Compare both sides of the tooth profile. If there is a significant difference in the shapes, contact your manufacturer. If the float is within specifications, visually inspect for surface distress. Catching roller bearing damage early can minimize related damage.
Every system has a critical range of operation either below or above normal operation speed.
A torsional problem will produce an erratic noise and may show up at a new start-up or after system upgrade. If suspected, contact your manufacturer. Today, there is a variety of good monitoring equipment which can measure noise and vibration levels. It's designed to provide hard copies or vibration signatures which can establish a history or base line for your drives. Once again, when all other checks prove negative, do not hesitate to contact your manufacturer for assistance.