Probably the combination of the exhaust flow and OEM exhaust piping passing under the small cavity inside the boot (hidden compartment under the trunk felt pad above the battery compartment) causes the droning sound at some rpm level. Some 1M owners suggested to fill the cavity with non heat sensitive sound deadening materials. However, no info yet re heat canalization safety, hence, better leave it 'as is'.
See:
http://www.1addicts.com/forums/showthread.php?t=759270
Also the replacement of the N54 midpipe by an alternative one (such as the N55, Maddad or Berk Technology midpipe) and/or the replacement of the 1M exhaust by some aftermarket exhausts (such as the 1M Akrapovič exhaust) appear to significantly reduce the drone phenomenon.
See:
http://www.1addicts.com/forums/showthread.php?t=651547
http://www.1addicts.com/forums/showthread.php?t=662265
If I were you, I would re-install the OEM downpipes and focus on midpipe (N55 or Maddad) and axle back (after market slip-on rear muffler) to get rid of most of the drone nuisance.
An example (1M Akrapovič slip-on exhaust + N55 mids):