I have a 2001 f150 84k, I seldom ever use the 4wd switch since I bought the truck new in 2000. Yesterday I thought I would try it just to lube the seals (Not sure if that's true or not) When I switched going about 20mph there was a slight clunk, maybe about 3 times louder than the normal noise while switching to 4wd, this wasn't super loud but noticeable. There was not any noise after that and everything seemed fine. When switching back to 2wd there wasn't any noise at all. Any ideas what this might be? I would appreciate any input you guy's have. Thank-you.

 

So far you have not reported anything unsual.
The clunk normally is the front vacuum actuator operating to lock up the front.
You won't alway hear anything when it disengages.
Good luck.

 

I don't remember it ever being that loud? I'm I being too paranoid?