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.
2015 M3 6MT | Yas Marina Blue, CF Roof, Cloth | Adaptive M Susp, 437M 19" Silver | Stripper
2013 X5 xDrive50i | Carbon Black | M Sport + M Perf, 333M | Loaded