2004 F-150 makes noise in 2WD

My truck makes a ratcheting sound when accelerating in 2wd. I put it into 4wd, it stops. I am aware of IWE and solenoid issues. However, my truck is a manual shift 4wd. (No button. Must shift lever into 4wd) If I go into 4wd, then back into 2wd, the noise will subside for a very short period, then return. Almost sounds like a hub trying to engage. Even though the 4wd is manual shift, are there still vacuum lines to engage the hubs? This truck doesn't have a push button, a solenoid, ect. It is all direct shift.

The truck has a 5.4 with automatic tranny.

Thomas