Happy St Pats day Friends. 1999 F150 XLT 5.4 has decided to make a disconcerting ratcheting noise when shifting to 4 drive only on the fly. It will engage after 4 to 6 seconds with a healthy thump. Up till now it has performed flawlessly and made no noise whatsoever during engagement . I can't tell if the issue is originated with the half shaft axel engagement or the transfer case. Any ideas on what might cause with will be appreciated?

 

It could've been the actuator not engaging all the way.

 

Not sure if the electronic differential actuators are the same on a 99 compared to an 05 but mine started doing pretty much the same thing but when it was not moving, ratcheting sound then the big thump when it finally engaged. From the time I first noticed it until it finally would not engage was about a year and as usual it went out when I was in the middle of no where hunting. Finally made it back to civilization and took it to a local shop, $200 later with a new differential actuator we were back in business.