Chevrolet Repair: 305 v8 whistle

Question
Ok I have a 93 chevy 1500 v8 tbi p/u and every time I drive the truck long enough to get it to running temp it starts to have a high pitch whistle. When I'm driving the truck and press the gas the whistle gets softer. If I press the gas hard enough the whistle goes away, but after I let off of the gas it comes back. My dad and I have replaced the idle air control and rebuilt the throttle body and that has not fixed it. I think I is one of the fuel injectors because I did not replace those when I rebuilt the throttle body. It is not a vacuum leak because I already checked all possible hoses. Any suggestions before I replace both fuel injectors?

Answer
It is a vacuum leak ,most likely . You missed it. Get a squirt bottle and use water. spray anywhere suspect,and when the noise changes, you found the culprit. There are lines that have manifold vacuum as far back as the gas tank. There is supposed to be some hiss, as air gets sucked past the throttle plate.