The buzz will be your roof skin resonating on the spars sitting 2mm below it.
(well known fault with Tourers without sunroofs!!)
Many main agents replaced exhausts to no avail when these were new.
To confirm:
Try pushing the roof lining upwards just behind your head when vibration occurs.
The cure is to remove the roof lining and pack the spars with silicone sealer.
Half a days work.
Let me know your findings 