Vehicle grinds in park and in neutral grinding stops when put into gear. I started the vehicle after parking it for two hours, was driving it around and it was fine. It was -10F out side at the time. I did not drive it anywhere after the grinding started just put it in drive and R. I just thought it was cold so we left in drive for 5 minutes to warm up. Grinding noise was still there so we turn it off and left there. Not sure on type of transmission either a 6f70. Type of transmission on door tag reads "C." I did not check transmission fluid. I am think torque converter or main bearing any Ideas.