If you're experiencing noise from your 9-inch rear axle, it could be caused by worn bearings or resonance from the rigid suspension. Using EP90 gear
oil is standard for most 9-inch axles and should provide adequate lubrication for the bearings. Thicker oils like EP70/140 or EP140 may help reduce noise by damping vibrations, but they can increase drag and heat, potentially affecting
ball bearing performance. If the noise persists after switching oils, consider checking or
replacing the bearings, as worn bearings may contribute to the noise.