787 Electric Hydraulic Pump
 

Just spent over 30 hours riding on a Dreamliner and I was shocked by the 850 hertz whine on take off and landing. Did Boeing attach that unit to the floor to make sure it had a good sounding board? Curious as to how Boeing could mount it in such a fashion to cause that much acoustic coupling. Quite irritating. Anyone know how that unit is mounted and why it is so loud in the cabin?

most likely the center hudraulic pump or the 4 CAC for air condition depending where you were seating
https://cimg9.ibsrv.net/gimg/pprune....8c9dd291dd.jpg