As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - FL 14 - EN ROUTE AT FL370, CREW RECEIVED THE EICAS MESSAGE 'LE SLAT ASSY.' CREW ALSO FELT A 'RUMBLE' WHICH MAY HAVE INDICATED A SLAT MOVEMENT. FLIGHT RETURNED TO PHL AND LANDED WITH NO FURTHER INCIDENT. MAINTENANCE PERFORMED THE BITE PROCEDURE PER THE FIM AND REPLACED THE NR 2 OUTBOARD SLAT PROXIMITY SWITCH (S277).