air pockets in hot water line

I HAVE SMALL AIR POCKETS IN MY HOT WATER HEAT LINE CAUSING POPPING OR CLANKING IN THE BOILER. I HAVE FLUSHED THESE OUT TOTALLY SEVERAL TIMES OVER THE LAST 2 YEARS. FOR 5 OR 6 WEEKS THERE IS NO SOUND OR AIR POCKETS. MY SYSTEM HAS A FILLER VALVE WHERE THE WATER SOURCE COMES IN TO REDUCE THE PRESSURE OF THE WATER SYSTEM. CAN THIS VALVE BE BAD & OR STUCK CAUSING NO WATER TO TENTER THE SYSTEM AS NEEDED & CAUSING THE AIR POCKETS? i HAVE AN XTOL TANK WITH 15# OF PRESSURE. DESPERATE FOR HELP. THANKS!