High-Pitched Continuous Tone

My son bought last week a 2005 GMC Safari with 80,000 miles for his business. As soon as this vehicle is turned on and running and continue till turned off, there is a high-pitched continuous tone occurring from somewhere near the middle to the end of the vehicle. What could possibly be the source and how is it corrected?

When idling or driving? 4.3L?

If the car is off, and you turn the key to the On position (not start, just on), do you hear the whine? I’m thinking fuel pump.

Another vote for a failing fuel pump, unless the sound varies with road or engine speed.