84 Screech

[YZ250]

I've picked up a 1984 Chevy C10 about eight months ago and have been lurking on 67-72chevytruck.com website for around the same amount of time. I've read multiple post on how to lower my truck and decided to go with, one cut coil spring up front and shackles in the rear. The new acquired stance was very close to what I wanted, but wanting the rear lower for a 2/4 drop I went with the shackle/hanger mod. When I initially had the truck with just shackles in the rear I didn't hear any screeching at all. When I added the hangers for the 2/4 I get this horrible load screeching sound. The screeching is only on hard acceleration. I have air shocks on the back so when I add air to raise the truck about an 1" the noise will disappear. Any thoughts on where the screech is coming from?

thanks