Did you manage to locate the problem?
I seem to have a simmilar issue - a rumble/vibrating sound from 1600-1800 RPM, only noticable in 5th and 6th gear at around 80km/h, Under 60km/h and over 100km/h its ok. It is also OK at 80 in 4th gear (over 2k RPM).
I noticed the sound goes away immediately if I press the brake pedal a little.