[peterjon1]

I can't tell you the exact location of the vacuum lines because the placement of various pieces seems to vary from year to year. Basically there is a line (probably these are 5/32 in) from a tee on an engine vacuum line to a switch on the top of the transfer case. Depending upon year, there may be another vacuum switch mounted on the firewall. Then from the transfer case switch to the actuator for the front axle, located under the battery. What I think you are hearing is the encoder motor on the transfer case. When you select 4wd, the encoder motor moves the switch on the transfer case, sending vacuum to the actuator for the front axle, which pulls a cable to move a shift fork in the front diff. See if you have transmission fluid in the vacuum lines. That's a sure sign the transfer case vac switch is bad. Part is $10-15. It is not too hard to replace if you can get under there. My work area was about 4" deep in water, so I had a shop replace mine, they charged $36 labor.