Recently my 2003 has started making a noise in 2WD. Best I can describe the noise is it sounds like a tire going over the rumble bars on the side of the road.

I suspect it is some type of gear cluster not quite engaging sounds something like that.

Usually is present when the vehicle is first started, and in 2wd, if I switch to 4wd and back to 2 wheel drive it goes away, might come back, usually comes back if the vehicle is parked with the engine off for a while.

It does not depend upon engine rpm, noise is there even if vehicle is in neutral with engine turned off. It does not change sound if braking or accelerating or coasting or turning left or right.

I am wondering if I might have a vacuum engine problem where weak vacuum might be low upon first start up and sometimes at idle, causing an actuator not to pull or push something completly.

Once the noise starts, the only thing that makes it drop out is push 4wd then it goes quiet, or back to 2wd and is quiet.

 

Have a read through the link below and see if that describes what you are experiencing:
4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO

 

Thanks that sounds like where I need to check.

 

So I had a chance today to pull the battery and the shelf and get to the vacuum actuator, the cable moves easily and snaps quickly back in so I suspect the cable is not sticking or the return spring is not the problem.

Today early AM I am going to hook the battery up with jumper cables start the engine and see if vacuum is leaking and pulling the actuator partially.

If that is the case I then need to check and or clean or replace the vacuum switching unit.