Heres whats going on. About a year ago i changed my coils from stock to a pair of longer more rigid procomps. As expected, the ride worsened, but unexpected, a strange noise comes from the rig when i close the throttle. The best way i can describe it is i get up to about 60mph and then if i let off the throttle it makes a sort of grinding noise. I later learned that a good deal of weight in the rear dulled these sounds. So i carry an extra 400lbs in the back to soften the ride and quiet the noise, but the noise is still somewhat audible. I might just go back to factory coils. let me know what you think.

 

Check your rear pinion and your u-joints. The driveshaft is now wearing at a different angle, most u-joints will make noise when there is load on the backside of the wear side.

Last summer I had a u-joint go out, it started off chirping when I let off the throttle, then moved into a heavy clicking/grinding/chirping noise untill I figured it out and replaced it.