My 2010 Equinox has developed a strange new noise. After I start the car and drive a couple of miles, there is a fairly loud, low toned, rhythmic thumping and vibration that seems to be coming from the rear end. It will do that for a couple of miles and then quiet down. I put new tires on the back and it did not help. Any ideas? I've had the car less than 2 years and have put thousands into it in repairs. I am beginning to hate it.