* Worn bearings: This is the most likely culprit. Over time, the bearings that allow the fan motor to spin smoothly wear down. As they become damaged, they can cause a thumping or grinding noise, especially when the motor first starts and is under initial load.
* Loose or damaged fan blades: If a fan blade is loose or slightly bent, it could be hitting the motor housing or other internal components, creating a thumping sound.
* Something lodged in the fan: A small object, like a leaf or debris, might have gotten into the fan housing and is now interfering with the fan's rotation.
* Winding issues (less likely): While you've ruled out the relay, there's a small chance that a problem within the motor's windings themselves is causing an intermittent electrical surge that's manifesting as a thumping noise. This is less common than bearing issues.
Troubleshooting steps:
1. Access the blower motor: This usually involves removing some parts of the dashboard or under-dash panel. Consult a repair manual for your specific truck model to find the location and proper removal procedure. Safety first: Disconnect the battery's negative terminal before working on any electrical components.
2. Visual inspection: Once you have access, carefully examine the blower motor and its components. Look for:
* Obvious signs of damage to the fan blades.
* Any loose parts or debris inside the housing.
* Excessive play or looseness in the motor shaft (indicating worn bearings).
3. Listen closely: With the power still disconnected, try to manually rotate the fan motor shaft. If you feel significant resistance or hear grinding, it's a strong indication of worn bearings.
4. Test the motor (with caution): If you're comfortable working with electricity, you can carefully test the motor using a suitable power supply (after reconnecting the battery) to see if it runs smoothly. Do this briefly and only if you know what you're doing, as incorrect voltage or polarity can damage the motor.
Likely Solution: If the inspection reveals worn bearings or a damaged fan, you'll likely need to replace the entire blower motor assembly. This is often a more cost-effective solution than trying to repair the motor individually. A new motor is usually readily available at auto parts stores.
