Hello, my name is Thomas i am new to the forum, i bought a 1999 F-350, with a 6 speed manual transmission, recently and love the truck. My problem is that when driving down the road it makes harsh humming noise in when in the higher gears (3rd, 4th, and 5th) and I feel it in my feet and my butt. The noise only last at most a second or two then goes away i have found no rime or reason to it yet. At first i though going over a bump caused it, but i have now felt it just driving down the highway. Any thoughts will help! Thank you.

 

Hmm I Know when I Had my 2000 powerstroke it would make a noise and shake the whole truck for a few seconds found out it was a bad hub and warped rotars so check them out also check your carrier bearing and U- Joints

 

i would check the carrier bearing.