It's drivetrain related due to the extra torque. Mine does it, too, sounds weird but is perfectly normal.
Anything you do to stiffen the subframe mounts will transmit more noise to the cabin.
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350