I have a humminig that sound as if it is coming from the rear passenger side, Has anyone experienced that prior to axel breakage?

every once in a while there is a Loud....GRRRRR from the back of the van.

talked to a mechanic and since it does not do it all the time he was not 100% sure but says it is probably the motor in the rear air shocks going out. Another mechanic says he has a different make of vehicle that has the same problem...