Hi there, I have a 2007 328i with 79,000 miles that I recently purchased. I have just started noticing that when I am sitting at a red light with my air conditioning running, a strange shifting vibration under my seat. My wife also noticed it on her side as well. It feels like something is shifting into or out of position. When I turn off the air conditioning, it does not happen. Anyone have any idea what this could be-thanks!