It also can be resonating sound from the frame, being tied in solid it will pick up and amplify any squeeks from under the truck too. I have found that using floor plates on the downtubes and another on the frame tie-ins to sandwich the body. Use different size plates with rounded corners and rubber bushings between the body and plates, the different sized plates will prevent tearing the stock floor pan.

 

I saw a similar type setup on a dakota and thought it was a real good idea. I think you are really on to something here. Hmmm. Thanks