2002 Buick Rendezvous whining noise and vibration on accele

My truck has been making a whining type of sound from the rear end when turning for the last few months. I noticed it a few days after I bought it from a private party. It makes the noise when cranking the wheel all the way to turn in a parking spot and sometimes when a turn a corner sharp. The more I turn the wheel the louder it gets. My mechanic cant figure out what it is but thinks it has to do with the awd. He said it sounds like rubber rubbing together. I have also noticed a vibration noise that I think may be coming from the rear but im not sure. It makes the vibration when going from a stop and accelerating to about 40 then its stops once I get to 40. What is it? I was told its not unsafe and to wait and see what happens and leave it alone for now. I want to get rid of it if its something major.