Problem fixed. The rear deck was banging down in the corners. The rear shelf sits on plastic lips on left and right. Either BMW or previous owner had stuck on under the shelf some thin sticky backed felt , which had moved. So replaced with some sticky backed foam pads used to prevent hi-fi, etc from scratching surfaces.
http://www.flickr.com/photos/75310855@N03/8557503525/
A is rear deck, B shows the rubber foam pad.