Okay, here it goes. 97 F150 electric shift 4x4.

Recently while driving down the road at about 55 mph my truck began to get a clunking noise. When I am stepping on the brake and it clunks I can feel it jar the truck and feel it through the brake pedal. At first I figured I may have had a bad vacuum line going to the front differential so I checked it and everything was okay with a decent amount of vacuum at idle. I unhooked the vacuum line to engage the front diff leaving the disengage line hooked up. Still clunks occationally. Today we got a bit of snow, so a drove around in 4wd to see if the transfer case was maybe trying to engage. The clunking continued. The fact that this clunking is totally random and not related to any bumps or when brakes are used, but really affects the truck when the brakes are used leads me to believe it is somewhere in the drivetrain, but I can't figure it out. Sometime when it is real bad I can feel it in the floor while driving. Some days it doesn't hapen and other times it will do it 20 times in a 10 second period.

 

Try the two bolts that are on your brake caliper. Sounds exactly like the problem I was having. They have vibrated loose or you are missing one, if that is the problem. Hope this helps you.

 

I had the same noise. I have a 2005 Supercrew. It would sound like something feel off the bottom of the truck and like a empty hollow can was dragging underneath the truck. I slowed up to 10 mph and it sounded like someone slammed a fist underneath the truck. I brought the truck in and It turned out to be somekind of electrical problem where the truck was trying to engage into 4wd. This must be a big problem. My dealer told me that the part is on national back order. And it would take a week to get the part from ford.