Symptoms
- Whining or humming noise from underneath that changes with speed
- No record of differential service ever being done
- 4WD or AWD vehicle with unusual drivetrain noise on turns
Likely causes
- CommonInterval simply overlookedDifferential and transfer case fluid isn't part of a typical basic oil change.
- PossibleFluid contaminated with metal wear particlesNormal gear wear puts tiny metal particles into the oil over time, which increases wear if never changed.
- Less commonWater intrusion from deep water crossings or off-roadingSeals can let water in, which turns the oil milky and reduces protection.
What to do
- Check your manual for whether your vehicle has a serviceable differential/transfer case and its recommended interval.
- Listen for a whining or humming noise from underneath the car that gets louder with speed, which can indicate low or worn-out gear oil.
- Have the fluid checked for level and condition; it should not look milky, metallic, or burnt.
- If overdue, have the fluid drained and refilled with the specified gear oil, which is different from engine or transmission oil.
- Ask the shop to check the fill and drain plug magnets for excessive metal shavings, which can indicate wear beyond just needing fresh fluid.
- Note the new service date for your next interval.
Tools and parts
Manufacturer-specified gear oilSearch: “differential gear oil for [your car year make model]”
When to call a pro
If the whining noise is loud, gets worse quickly, or you feel vibration, have a drivetrain specialist inspect it soon rather than just changing fluid.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.