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