The synchronizer shaft for the camshaft position sensor can cause a loud squeak or chirping noise due to failure of the upper bushing. The noise may be erratic and is usually present when the engine is warm.
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Repairpal verified 1997 Ford Aerostar problems
If you experience poor idle quality and fuel economy (and the Check Engine Light illuminates) after the exhaust system or motor has been removed and replaced, it could be that the oxygen sensor wires are crossed at the rear of the motor. Wiring schematics should be checked to ensure that the correct harness plug is connected to the corresponding sensor.