[Jimmy_Brooks24]

Getting a weird shaking in the floor boards bad enough to feel it in the seats. It only happens under harder acceleration at 80+ MPH. When I rev the car at a stand still there is no shaking, and when accelerating hard from lower speeds there is no shaking. The steering I wheel doesn't shake so it's not likely to be unbalanced wheels. I checked under the car and all the suspension bushings look good. It almost feels like it's coming from the transmission or drive shaft. Bad transmission mounts? Could it be differential related? It's a 2012 535i M-Sport RWD with 77k on it, completely stock. Anyone have any idea?

[7'er-addict]

That sounds to me like one of the guibo flex discs.

Check for cracking.

Or pull down the heat shields above the exhaust and check the center support bearing. There should be no play up/down side to side.