Why does my F-150 clunk when I reach the speed of 30MPH and grind in the front driver wheel ?

Question

When i reach the speed of 30 MPH the truck makes thunk and grinding sounds , but when i slow down to a speed lower than 30 MPH the noises stop. (1986 Ford F-150 4X4)

Answer 1

Best place to start would be wheel bearings.  

Answer 2

I bet you have auto locking hubs.  find a good 4x4 mech and have them check before you find yourself in a ditch or worse