* A/C Fan Motor: The A/C system's blower motor is located near the air filter, and a failing bearing within the motor can create a loud humming or whining sound. The increased heat and the extra load placed on the motor when the A/C is running exacerbate the issue. This is the most likely culprit.
* Auxiliary Fan: The auxiliary electric cooling fan, also located near the radiator and air filter, helps cool the engine. If the fan's motor bearings are worn, it can produce a humming noise, especially when the engine is hot and the fan is working hard.
* Loose or Damaged Components: While less likely, a loose component near the air filter housing could be vibrating and creating the humming sound, especially under the increased stress of heat and A/C operation.
Troubleshooting Steps:
1. Listen Carefully: Try to pinpoint the exact location of the hum. Is it definitely coming from the air filter area? Try turning the A/C on and off to see if the noise correlates.
2. Visual Inspection: Once the engine is cool, carefully inspect the area around the air filter and the A/C blower motor. Look for anything loose, damaged, or obviously worn. Check the auxiliary fan blades for damage or unusual play in the motor.
3. Listen for Noise with A/C Off: Does the hum happen when the A/C is *off* but the engine is hot? If so, it points more towards the auxiliary fan.
4. Feel for Vibration: When the noise is occurring, gently touch components near the air filter (while being mindful of hot surfaces). A strong vibration could indicate a failing motor.
If you determine it's the A/C blower motor: Replacement is usually necessary. It's a relatively common repair, but access to the blower motor can be tricky depending on the Explorer model.
If it's the auxiliary cooling fan: Similar to the A/C blower motor, replacement is often the solution. Again, access may vary.
Important Safety Note: Always allow the engine to cool completely before working on it. Be cautious near moving parts and hot surfaces. If you're not comfortable working on your car yourself, it's best to take it to a qualified mechanic for diagnosis and repair.
