Weird issue truck accelerates faster at half throttle??

[yellow400ex05]

I've got a 1984 chevy c10 with a 427 bbc 780cfm quickfuel carb it runs on the rich side but I noticed while on the highway tonight that it'll accelerate much slower when you mash the pedal to the floor and when you gradually let off Til half throttle you can hear a whistle or a hiss sound sometimes and it will accelerate much faster at 3/4 throttle than full throttle. I have no cats and a fresh fuel filter as well.