A couple of thoughts...the GF kits rides siffer could have meerly exposed / amplified an existing problem.
Or....
There are body seams on the uinderside of the bed. Does the 2/5 kit use the extended shackles? If so, the bed seam lips on the underside need to be bent away from the leafspring rear eyelits for clearance. If that has not been done you will most certainly have a rattle until you hit a large enough bump to crush or bend them away. Take a quick look under the bed.
2006 Harley Davidson Ford F150 - SOLD
McGaughy's 2/5 Lowering Kit, Air Lift Slam Air Assist (Rear), Roll Pan, 4.10:1 Gears, Truxedo LoPro Tonneau Cover, Carbon Window Tint, Roll Pan w/Hitch
STS Remote Turbo System, Dirty Dog 11" Converter, FTVB Calibration Kit, Deep Aluminum Pan, Auto Meter Gauges, SCT XCAL2 Custom Tune, 434HP 511TQ
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Denim Blue, PCB Black Inverted Front End, V&H Big Radius, SERT, SE Air Cleaner Kit, HTCC 10.5:1s, HTCC CNC Ported Heads, 103" Cylinders, 251 Cams, 97HP 104TQ
2008 Cargo South 7x16 V-Nose Enclosed Trailer
Black w/Diamond Plate Stone Guards, Aluminum Mag Wheels, White Interior Walls, Black & White Vinyl Floor (In Process), E-Track in Floor (In Process)
2007 Shelby Mustang GT #4453
No Modifications - Yet!!!
2007 VW GTI 2.0FSI (Wifey's Toy)
Revo Stage I, Progressive Tint, Forge Motorsports Billet Diverter Valve
Ford Taurus (Winter Driver)