What was found
AIRCRAFT WAS GROUNDED. DURING CRUISE, CREW REPORTED A CHATTERING SOUND IN COCKPIT, ACCOMPANIED BY NUMEROUS AUDIBLE AND VISUAL WARNINGS. APU WAS STARTED, LT ENGINE DRIVEN GENERATOR SELECTED OFF AND AC BUS X TIE, OPENED. EMERGENCY WAS INITIALLY DECLARED BUT SUBSEQUENTLY CANCELLED. MAINTENANCE REINSTATED THE LT GENERATOR AND FOUND THE FREQUENCY TO BE OUT OF LIMITS. ADJUSTED FREQUENCY IAW AMM 24-20-00 PARAGRAPH C. OPS CHECK NORMAL.