Well, after taking the Eisenmann off because of the louder than expected volume, I looked more closely at the connection between muffler and connecting pipes, and on one side the connecting pipe is out of round and too small.
This was causing my small exhaust leak, and I'm sure was causing additional vibrations and drone since I can move the connecting pipe freely even with the clamp tightened to the maximum.
The other side is fine.
Not sure who made this connecting pipe, but I contacted my original reseller to get a replacement, will keep you guys updated.
2013 X5 xDrive50i | Carbon Black | M Sport + M Perf, 333M Wheels
Sold: 2012 E92 M3 | Jet Black, Fox Red/Black | DCT, ZCP