To check the oil in your Gear Box, drain out a small sample and check it for sludge, water, or metal. Replace any seal with oil leakage around it. Check the oil level and replace it if necessary. Make sure all hardware is tight and in good working condition. Check that the outside of the case is free from excessive deposits. The backlash can be checked by rotating the pinion shaft back and forth and, before the gear teeth engage fully, noting the amount of free rotation. The endplay can be checked by noting the amount of movement in the Gear Box output shaft when pulling up and down on a fan blade tip. While some backlash and endplay are necessary for proper gear life, excessive backlash and endplay mean it is time to replace the Gear Box. When in doubt, contact a qualified service contractor.