I love my 2004 LS 6Cyl Basic, 2 owners, 32,000 miles. Everything works but: If the car sits for a few days (as it frequently does) I have a vibration from the rear end. It is a rumble to a shimmy. It goes away after driving the car for 15-30 minutes. I have new Michelins and TSW wheels. The problem does not move after rotating wheels. The TSW wheels have the centering rings installed. Car has never been wrecked and is garage kept. The under-carriage is literally like new. The longer the car sits the more significant the vibration is. I am stumped. Help, please.