* Alternator: A failing alternator bearing is a common culprit. The hum will often change pitch with engine speed (though it might be subtle at idle).
* Power Steering Pump: A worn bearing or internal problem in the power steering pump can produce a hum, especially when the steering is slightly turned.
* Belt Tensioner/Idler Pulley: A worn bearing in the belt tensioner or idler pulley will often create a humming or whining noise. This is often more pronounced at higher RPMs but can be heard at idle.
* Water Pump: Though less common, a failing water pump bearing can also contribute to a humming sound.
* Wheel Bearing (unlikely at idle): While typically associated with driving, a severely worn wheel bearing *might* create a faint hum even at idle, especially if the vehicle is slightly rocking.
* Exhaust Leak: While usually a deeper rumble or drone, a leak near the engine could resonate and create a humming quality at idle in some cases.
Troubleshooting Steps:
1. Listen Carefully: Try to pinpoint the location of the hum. Is it coming from the front, side, or rear of the engine? This will help narrow down the possibilities.
2. Inspect Belts and Pulleys: Visually inspect the serpentine belt and all pulleys (alternator, power steering pump, water pump, AC compressor, etc.) for cracks, wear, or any play in the bearings. Try to gently wiggle each pulley – excessive play indicates a worn bearing.
3. Check Fluid Levels: Make sure the power steering fluid level is correct. Low fluid can lead to increased pump wear and noise.
4. Listen with Engine Off: Some sounds might be present even with the engine off, helping to isolate the source further.
If you're not comfortable performing these checks yourself, it's best to take the Aerostar to a mechanic for diagnosis. Given its age, several components might be nearing the end of their lifespan, making a thorough inspection crucial.
