Just bought my 2000 F150, 4x4, ext cab, 4.6...has a new transmission and runs and drives very nice. However, testing my 4x4, with the truck in neutral, brake on, switch it to 4LO, it has a VERY LOUD CLANK!!! First thought maybe a very worn ujoint, but no...then I thought it was the vacuum actuator, but realized it is only a plastic device to push the arm. This is absolutely a LOUD clank clunk npoise, but it goes into 4LO just fine and works good, and will deactivate well from 4LO...does not do it in 4HI.

I have read a lot of posts that say a clunk is normal, but it sounds really loud. Any idea, or am I just imagining how bad it seems and it really is normal...

 

check fluid level on tcase

 

Gonna change the fluid in a few days and will see if it corrects the problem. I thought maybe it was a UJoint, so got underneath and had my wife put it in 4LO so I could look, and it was definitely not the UJoints on the driveshaft. I was on the driver side so I could watch and listen for it, but the sound seemed to be on the passenger side. Will get under again after changing the fluid and have her do it again if the fluid does not correct it.