Help! Having a whistle noise coming from engine

Two days ago, I heard a high pitch whistling sound coming from my engine (I believe). It only made the sound when my foot was on the accelerator and would stop if I hit the brakes. Yesterday, my truck was fine and there was no sound. And then today it was making the sound again. Would someone please tell me what this sound means? I have a 2003 Toyota Tacoma 2WD 4 Cylinder Automatic.

 

I have the same problem in my 2012 regular cab 4 cyl manual 4x4. When I perss the accelerator barley around 60 mph it does it. Haven't been able to figure out what is causing it yet.