HOW DO YOU KNOW IF INJECTORS ARE BAD ON A POWERSTROKE DIESEL ? DO THEY MAKE A NOISE ? READ MORE DETAIL PLEASE?

----- MY TRUCK IS MAKING A HUMMING NOISE WHEN I TURN THE KEY ON, TO TURN THE GLOW PLUGS ON AND IS CONTINUING IT ALL THE TIME.. ALSO HAVING PROBLEMS STARTING.. MAKES A CLUNK SOUNDS WHILE STARTING SOMETIMES.. ON A 7.3L POWERSTROKE DIESEL.. 2002 FORD F250. -- THANKS -