strange noise when accelerating in 3rd gear 2000 v10 f350
 

https://drive.google.com/file/d/1d26...EEXDBsjj5/view
click above link for a video clip to hear noise.
this noise is not constant, it only happens maybe 1 out of every 3 drives when empty, but more common when hauling heavy load. ONLY happens when accelerating in 3rd gear, or when going uphill or when loaded. stops when i let off gas. occurs around 1800-2100 rpm when empty. sometimes when i hear the noise it seems im not accelerating quite as well as i should be. stops as soon as i shift into 4th. definitely coming from the front of my truck. 2000 v10 f350 automatic with 90k miles.

ive heard 2 theories; vacuum leak or failing u joint.