k1500 96 sliverado

Im very stumped on this; i have gone thru 2 actuators for the 4x4 in the past
9 months. But what has me super confused is the fact that when i either
drive take off or go in reverse theres this really loud sound that occurs
randomly it can go for days with out happening or once every few hours
sounds almost like the car is trying to engage 4x4 without the button being
pressed or the actuator only moves half way or is getting stuck without the
button being pressed the noise sounds like something rubbing terribly wrong
its gets louder an faster as i speed up stops once the cars stops but makes a
loud thud when i come to a complete stop but while making that noise if i
were to press the 4hi button an switch it back to 2hi it stops really confused
on whats going on any help would be great