2001 3500 4wd Knocking When Engaged

[ognir0045]

My gmc 2001 3500 is knocking when 4wd in engaged.

 

When I am in 2wd it does not knock. As soon as I hit the 4wd button I hear a grind. My passenger side engages and my drivers side is not. about every 25 feet or so the drivers side knocks. kinda sounds like it's trying to engage but it can't. I am going to pull the actuator and check it out.

Is there anything else I should be checking?

Has anyone had this problem?

 

And like I said when I am in 2wd the truck runs nice and smooth.

Ther has been no mods to the suspension, stock tire size, 8.1 liter with alison trans, 145 000km

 

Thanks for any help

Dave