* Worn Blower Motor: This is the most likely culprit. The blower motor's bearings might be worn out, causing noise and reduced power at lower speeds. At high speed, the motor's stronger force might overcome the friction from the worn bearings, allowing it to work, but noisily.
* Faulty Blower Motor Resistor: The resistor controls the speed of the blower motor. A failed resistor can prevent lower speeds from functioning correctly. The high speed often bypasses the resistor, allowing the motor to run directly from the battery. This wouldn't necessarily *cause* noise, but it could exacerbate existing bearing noise if the motor is already worn.
* Worn Blower Motor Cage: The blower motor's fan cage might be rubbing against something inside the housing, causing noise and restricting airflow at lower speeds.
* Obstructed Air Ducts: Dust, debris, or even leaves can clog air ducts, restricting airflow. High speed might be powerful enough to overcome this obstruction, but lower speeds wouldn't.
* Loose Blower Motor: A loose blower motor can vibrate and cause noise.
* Electrical Issues (Less Likely): While less probable given the symptoms, a problem with the wiring to the blower motor or its speed control could contribute. However, a complete electrical failure would usually result in no functionality at any speed, rather than just limited speeds.
Troubleshooting Steps:
1. Check for Obstructions: Inspect the air intake and any visible air ducts for obstructions.
2. Listen Carefully: Try to pinpoint the source of the noise. Is it coming from the blower motor itself (under the dash)?
3. Inspect the Blower Motor Resistor: Locate the blower motor resistor (often a small, heat-sinked component near the blower motor). Visually inspect it for burn marks or other damage.
4. Test the Blower Motor (Advanced): This might require removing the blower motor to test it directly with a power supply (be cautious when working with electrical components). This would confirm if the motor itself is faulty.
Recommendation:
Given the age of the vehicle, it's likely a worn blower motor or resistor. It's best to have a qualified mechanic diagnose the issue. Replacing the blower motor is relatively straightforward, but access to it can vary depending on the car's design. A mechanic can properly diagnose the problem and ensure the repair is done safely and correctly. Replacing the resistor is also a relatively easy task.
