My truck is a 2001 F-150 4x4. There is a noise that has developed in the last couple of days. When driving at about 25 mph or faster a knocking sound comes from under the truck. It's occurs randomly, and does not seem to change pitch with speed. It sounds like it is under the mid to rear of the truck not under the hood at all. I does happen more often the faster I drive but it is not consistent at all. My wife said it sounds like a bungee cord hanging off the truck somewhere and banging into the side or in the back banging intothe bedliner. There is no bungee cord. I rode in the back for a couple of miles to listen to it and it sounds a lot worse than a bungee cord! I was actually getting a little worried back there with a loud knocking noise beneath me and my wife driving my truck at what seemed like an ever increasing speed!

I checked the U-joints and they seem good to me. There is no play in them that I can feel. I even jacked up the rear where I could turn the tires and the joints are tight with no noises. I do here a slight metallic rubbing sound from the rear wheels when they are turning. It sounds like the brake pads touching the rotors though. No crazy grinding sounds there.

Could it be that the U-joints are bad even tho I can't feel any play in them? Could it be something else that I haven't thought of. Any help would be appreciated. Thanks in advance!


UPDATE: I just put the truck in drive with the rear wheels jacked up off of the ground. I ran it up to 45mph and never heard the knocking sound. I could hear the metallic rubbing sound but it was not very loud. I think the rubbing sound has been happening for a long time, thats why I think it has nothing to do with the knocking.

 

Just a thought.. Try tightning the lower shock mount bolts on the rear. I've seen that cause a noise like that more than once.

 

Here is a dumb question. Is it all wheel drive? if so are all the tires the same? I have 1999 navigator with all wheel drive. My knocking came from the different type tires it had. Cost me 200 dollars at the dealer to figure it out.

 

Thanks for the responses.It turned out to be the rear wheel bearings were bad. I knew what it was when my wife told me she walked by the back of the truck and smelled burnt grease. She said she smelled right by the rear passenger tire and she almost threw up. I took it to a mechanic because the rear wheel bearings are not something I wanted to deal with. They set me up with new bearings and the problem was solved.