I had this on one of my 7 series BMW's. This is how I found out what it was....
To eliminate the sunroof, or indeed indentify it as the culprit, put some masking tape round the edges of the roof so that you are in fact taping it shut. Then run up to 60 and see if it's gone. If so, you have the culprit, if not, then you know it's not the roof.
May sound silly, but it sorted the 7.
Thanks
Mike