2000 Chevy Silverado loud whinning sound

Here's yr second oppinion. Take the bed off and replace the pump! They're known for being loud when they're starting to fail. Those engines are really picky when it comes to fuel pressure. It has to be at 65psi or it'll run bad or not at all. It's going to go out soon so replace it now before you're stranded on the side of the road.

Hook a fuel pressure guage to the service port when you're planning to let it sit for a while. Watch the guage while you attempt to start it for the first time. See if, or how long it takes to build the fuel pressure. It should ALL be there the first time you turn the key on. Meaning, it should build 65psi when you turn the key on. If not, turn the key back off without attempting to start it. Turn it back on and watch the guage. Did it achieve the pressure? If it takes 2 or more key cycles to get the pressure, we need to do some more diagnosing. You've fixed the whine that was the original complaint, am I correct? Was it a new fuel pump? Those pumps have a built in check valve that prevents the fuel in the rail and lines from draining back to the tank. If the valve is bad, and the fuel DOES drain back, it requires a little extra spin time before starting. There's also a possibility that you have an injector leaking, causing the fuel to drain through it, thus causing the same results. Do the pressure guage test I mentioned and we'll go from there. Hopefully you didn't get a bad pump but we have to know what the pressure is before we go much farther.

OK, since it doesn't build the correct pressure the first key cycle, I can't really blame the check valve for your problems. The check valve is internal and not visible from outside the unit. This valve doesn't have any thing to do with how much pressure it makes while the pump is running. Either the pump is faulty or you have an injector stuck open. I just re-read your post. It really sounds like the pump is just too weak. Did you replace the pump yourself? If so, did you replace the filter/sock on the bottom of the pump?(Inside the tank) Also, did you REPLACE the little rubber hose between the pump and the metal line?(Again, inside the tank) If the hose has a hole in it or is leaking in some other way, you're going to lose pressure at the injectors. If it's a small enough leak, you'll eventually, after a few key cycles, get a good bit of fuel pressure but still probably not enough. Hmmm. We may be on to something. Even if it was done at a professional shop, this could still be causing a problem. I think I would have that checked first.