It could be you can hear a noise coming from a ‘whining’ gearbox. This is noticeable at any speed, any gear, any time. If there is too much endfloat in the output shaft gears will rub inside. It’s not dangerous but is avoidable at a rebuild. By using thicker shims the float can be reduced from a noise inducing.020″ (0.5mm) to .008″ (0.2mm).