Truck is an 03 F350 with the 6.0. It sometimes sounds like the shift motor on the transfer case is trying to engage. It has the vacuum system which is blocked off. Warn aftermarket hubs. Switch on the dash with 2wd, 4wd and 4 low.

It's kind of an off and on thing. Try to engage for a few or more seconds then nothing for awhile then repeat. It sounds like it's at the transfer case. It could very well be coming from firewall/engine bay area as well. Is there anything between the dash switch and the motor?

 

so are you trying to move it into 4x4 and that is why you are getting the sound? There is are fuses but that should be it.

 

It does it all on its own. It sounds like it is trying to engage but doesn't. I did some research last night and there is a module between the switch and the motor. That would tell me it is either the dash switch or the module.

4wd works as it should when the switch is in the 4wd position.