* Wheel Bearings: This is a very common culprit for humming noises, especially at higher speeds. A failing wheel bearing will often make a humming or growling sound that increases with speed. This is high on the list of suspects since you've ruled out tires and rotors.
* Transmission: While less likely to be a *humming* sound, a failing transmission can produce whining noises, especially under load. This is more likely to be speed-dependent and might change pitch depending on the gear.
* Power Steering Pump: A failing power steering pump can make whining noises, especially when turning the steering wheel. The noise might change in intensity depending on steering input.
* Alternator: A failing alternator can sometimes produce a whining sound, often related to engine speed.
* Belt Tensioner or Idler Pulley: While usually a squealing sound, worn bearings in these components can sometimes create a humming noise.
Troubleshooting Steps:
1. Speed Dependence: Does the humming change pitch or intensity as you increase or decrease speed? This is crucial for diagnosis. A consistent hum at all speeds points more towards the transmission or a drivetrain issue, while a speed-related hum points more towards wheel bearings.
2. Turning Test: Does the noise change when you turn the steering wheel? If so, power steering is a strong suspect.
3. Jack Up and Spin Wheels: Jack up the front of the car one wheel at a time. Spin the wheel by hand. Listen carefully for any unusual noises. A rough or gritty feeling, or a humming sound from the wheel bearing itself, indicates a problem. Pay attention to any play in the wheel as well; excessive play is another sign of a bad bearing.
4. Inspect Belts and Pulley: Visually inspect the belts and pulleys for any obvious wear or damage. Check for any wobble or play in the pulleys.
5. Professional Inspection: If you're not comfortable performing these checks yourself, take your car to a trusted mechanic. A proper diagnosis requires a trained ear and potentially some diagnostic tools.
It's important to get this checked out promptly, as ignoring a humming noise, especially if it's related to wheel bearings, can lead to more significant damage and potential safety hazards.
