1993 GMC Sierra 1500 4.3L V6 makes loud whistling noise once engine warms up. Accelerating sometimes stops noise, idling or engine in park does not stop noise. WHAT IS CAUSING NOISE?

SOURCE: Heating System

The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.
Fig. 1: View of the Engine Coolant Temperature (ECT) sensor Fig. 2: Engine Coolant Temperature (ECT) sensor locationTESTING
See Figures 3 and 4

1. Remove the ECT sensor from the vehicle.
2. Immerse the tip of the sensor in container of water.
3. Connect a digital ohmmeter to the two terminals of the sensor.
4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
6. If the sensor does not meet specification, it must be replaced.

SOURCE: whistling Jeep grand cherokee 1999

Sounds like a vacuum leak. Check that all your vacuum lines are attached and in good condition.

SOURCE: loud whistle noise

check out your power steering fliud and pump. make sure all of you belt driven motors are tight.

SOURCE: oil pan removal on 2002 gmc sierra

did you pull the rubber plugs between the motor and trans out? there are either nuts or bolts up there

SOURCE: makes a loud whistling noise after accelerating

Well, unfortunetly, a lean code with low fuel pressure and a whining fuel pump all point to a failing fuel pump, which will leave you stranded. My suggestion would be to talk to the manager of the repair shop and have them work you a deal because from what you're telling me it sounds like they missed the ball on your first two visits. I would demand they not charge for the labor, so you just pay for the part. Let me know if you need further explanations.