I have a 2006 550i E60 and in the morning when I start up I hear a rattling sound when I am driving thru my condo complex. When I hit the main road it either goes away or I can't hear it. I still hear it a little bit later in the day, but it is not as pronounced. I took it to a shop and they couldn't hear the noise. Any idea what it could be? 54k miles. Thanks