1999 Chevrolet S/T Pickup

RATTLE / BUZZING FROM CSFI FUEL INJECTION SYSTEM, 4.3L V6, ENGINE CODE X. THE INLET AND RETURN LINES BUZZ WHILE ENGINE IS RUNNING, REGARDLESS OF RPM. SOUNDS LIKE THERE IS SOMETHING LOOSE UNDER THE IP / DASH IN THE INTERIOR COMPARTMENT. I TRACED THE SOUND TO THE FUEL LINES, WHERE THEY ARE MOUNTED TO THE INTAKE MANIFOLD. THE HARD LINES ENTER THE REAR OF THE CSFI UNIT AND THE BUZZING EMANATES FROM THIS POINT. THE VIBRATION CAN BE FELT IN THE ACCELERATOR PEDAL AS WELL. THE NOISE IS WORST ON A COLD ENGINE AND / OR WHEN THE OUTSIDE TEMPERATURE IS BELOW 80-DEGREES OR SO. THE NOISE IS ALWAYS THERE ON START-UP AND ON NON-HOT DAYS, IS CONSTANT. A FEW TIMES THE NOISE IS NOT HEARD AT FIRST, BUT AFTER 30 SECONDS OR SO OF OPERATION THE NOISE RETURNS. AGAIN, THE NOISE IS PRESENT AT ALL RPM, ALL DRIVING / RUNNING CONDITIONS. IT'S ALMOST LIKE A GURGLING SOUND IN THE INTERIOR COMPARTMENT, MAKES ONE BELIEVE THERE IS SOMETHING DANGLING UNDER THE DASH BOARD OR THERE IS AIR IN THE COOLING SYSTEM / HEATER CORE. THE VIBRATION / BUZZ CAN BE FELT THROUGH THE HARDLINES AND THE FLEX LINES. THE FUEL PUMP CAN BE HEARD BUT IT DOES NOT SOUND OUT OF THE ORDINARY. THE NOISE OCCURS NO MATTER WHAT LEVEL OF FUEL IS IN THE TANK, GRADE OR BRAND. I ALSO DETERMINED IT IS NOT A VACUUM LEAK, NO HOSES ARE TORN, ALL EMISSION EQUIPMENT IS CONNECTED CORRECTLY - THIS IS THE FUEL LINE / SYSTEM. THIS HAS BEEN GOING ON / OFF FOR THE PAST YEAR, ONLY NOW IT IS GETTING WORSE. I TOOK VEHICLE TO THREE REPAIR PLACES, EACH TIME BEIN