Thanks for you input, in answer to your questions, my Expedition is a 4WD,
it happens when in 2WD, I haven't tried to see if it does the same in 4WD.
It has 165000. miles on it, but has been well cared for. The noise comes
fron the middle, and sounds alot like a bad U-Joint, but I have had those
checked they are fine.
I'll try the Expidition owners Group. Thanks again for your help.