- Wheel bearings: Worn-out or damaged wheel bearings can cause a humming or grinding noise that gets louder as the vehicle's speed increases. When the vehicle is shifted into 4WD, the weight distribution changes and the load on the wheel bearings is reduced, which can cause the noise to go away.
- Differential bearings: Similar to wheel bearings, worn-out or damaged differential bearings can also create a humming noise that is more noticeable in 2WD than in 4WD. The differential is responsible for distributing power to the wheels, and when it is engaged in 4WD, the load on the bearings is reduced.
- Driveline components: The driveshaft, CV joints, and U-joints are all part of the driveline that transmits power from the transmission to the wheels. If any of these components are worn out or damaged, they can create a humming or vibration that is more pronounced in 2WD than in 4WD.
- Vacuum Disconnect: The vacuum disconnect is responsible for engaging and disengaging the front axle when shifting between 2WD and 4WD. If the vacuum disconnect is not functioning properly, it can cause a humming noise in 2WD due to the front axle being partially engaged.
- Tire Noise: Depending on the type and condition of the tires, they can also generate humming noises at certain speeds. If the humming noise varies with the speed of the vehicle and the noise disappears in 4WD, it could be a tire-related issue.
- Transfer case: The transfer case is responsible for transferring power from the transmission to the front and rear axles. If there are issues such as worn bearings or any internal components in the transfer case, they can cause a humming noise, particularly in 2WD mode when more torque is going through the transfer case.
To diagnose the exact source of the noise, it is recommended to have the vehicle inspected by a qualified mechanic or automotive technician.
