Sounds like they broke a fan blade.  Did you ever see a parked jet at the airport and noticed the big fan at the front of the engine slowly turning in the wind?  That's because the engines "free spool" as they don't have brakes.  So imagine an engine with a broken fan blade flying at mach ,78 or so.  That big fan is still free spooling at a high rate even if the engine is shut down, but the rotor is spinning  out of balance because of the loss of a fan blade, thus the vibration resonating through the aitcraft. .  Not a real safety hazard, but very scary.