Squealing noise in Diff lock

Hi I need help, The rear of my GQ patrol with Diff Lock , has a squeal noise like if you have blowen up a balloon and stretch the top and let the air out slow. It dos'nt happen all the time and when it does its lound. Only when you are moving either slow or fast forward or reverse. But not while the car is still with the motor still going. The variation in sound dosnt change in speed of vehicle from slow to fast.
It can go all day without doing any thing then the next day its like all the time. It can Comes and go any time. First thing in morning when cold or even after 1 hour drive. It has know given time, or for how long it stays on. To me It sounds like a air vacuum , The diff lock works, found that out when turned it on and slowly drove off and turned all ok I can feel it realy crab on the road. Ive changed brake rotors and pads thinking it was them. Then new vacuum hose's to diff and checked out fuel lines. When I walk slowly next to it whilst my missis drives it, it deffinatly comes from behind ( inside ) RH rear wheel, Ive also check out the fuel breather valve all ok and tried driving with fuel cap off, noise still there. Could it be the vacuum unit on the diff if so how do u check or fix ?? We have had a vacuum machine on last week it made no noise or change in pressure

Becoming realy annoying