I've got a 98 f150 ex cab 4x4, an yes i've used the search menu and came up with nothin.

When i drive normaly , by that i mean resonable acceleration and no bag driving, everything seems ok. however when i spin my tires in mud or try driving up a fairly steap slope theres a preaty loud noise. I can get it to make noise on the flat blacktop if its in 4x4 and i accelerate very quikly, but its quiet if i accelerate normaly. It seems to be coming from under my feet.

any help is appreciated??

 

in 4high or 4 low? floor shifter or or on the dash

 

it happens in iether hi or low and the shifter is on the floor.