A B757-200 ON LEVELOFF AT FL360 HEARD LOUD BANG UNDER CAPT'S SEAT. DIVERTED. MAINT FOUND LARGE SPRING BROKEN IN FORWARD E&E COMPARTMENT.

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WHILE PERFORMING THE CTL CHK ON TAXI OUT; I HEARD A SLIGHT NOISE AS THE CTLS WERE PUSHED TO THE FORWARD STOP. THE CTL CHK WAS REPEATED SEVERAL TIMES WHILE OBSERVING NORMAL INDICATIONS ON THE LOWER EICAS. WE PERFORMED A MAINT CHK ON THE AFT PANEL NOTING NO DISCREPANCIES. CHKED HYD QUANTITIES NORMAL. TKOF AND CLBOUT NORMAL. SHORTLY AFTER LEVELING AT FL360 A LOUD BANG WAS HEARD UNDER THE CAPT'S SEAT. FLT CTLS OPERATED NORMALLY. CLRNC TO RETURN TO ZZZ WAS REQUESTED. NO EMER DECLARED. FLT ATTENDANTS WERE BRIEFED THAT THE FLT WOULD BE ON THE GND I CHICAGO IN 30 MINS AND THE APCH AND LNDG WOULD BE NORMAL. NO CABIN PREPARATION. THE APCH AND LNDG WERE NORMAL. MAINT FOUND A LARGE SPRING BROKEN LOOSE IN THE FORWARD E&E COMPARTMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE AIRPLANE OPERATED NORMALLY AND NO FLT CTL CHANGES WERE NOTED. ON THE GND MAINT RECOVERED A BROKEN SPRING FROM THE FORWARD E&E COMPARTMENT AND RPTR BELIEVES IT CAME FROM THE ELEVATOR FEEL SYS. NO EMER WAS DECLARED.