AFTER DEPARTURE AND DURING THE INITIAL CRUISE CLIMB THE PLT AND PAX HEARD A METALLIC CLICKING DESCRIBED AS SOUNDING LIKE A 'VALVE TAP' IN AN AUTOMOBILE AND COMING FROM THE ENG AREA. THE PLT EXECUTED AN IMMEDIATE COURSE REVERSAL BACK TO THE AIRSTRIP WHILE STILL WITHIN THE NARROW GORGE OF THE SELWAY RIVER. THE IMMEDIACY OF THE REVERSE TURN WAS BROUGHT ABOUT BY THE PLT'S EXPECTATION OF IMPENDING MECH PROBLEM WITH THE ACFT AS WELL AS ANXIETY OF THE SITUATION AT SUCH A CRITICAL TIME DURING THE FLT. THE TURN AT A RELATIVELY LOW ALT WITHIN THE GORGE NECESSITATED A TIGHT TURN RADIUS AND THUS REDUCED THE RATE OF CLIMB IN AN ALREADY HEAVILY LOADED ACFT. THIS EVENT PLACED THE ACFT IN A POSITION WHEREBY CLEARANCE WITH THE HEAVILY WOODED UPSLOPING SIDES OF THE GORGE WAS NOT POSSIBLE. DISASSEMBLY, INSPECTION AND TESTING (WHERE POSSIBLE) OF THE ENG, PROP AND ASSOCIATED COMPONENTS REVEALED NO EVIDENCE OF ANY PREIMPACT MALFUNCTION WHICH WOULD HAVE RESULTED IN A POWER LOSS, NOR ANY EXPLANATION FOR THE CLICKING NOISE.