WHEN I FLUSH THE TOILET AND IT REFILLS IT WHISTLES. DOES THIS MEAN THERE’S AIR IN THE PIPES? IF SO, IS IT HARMEFUL AND WHAT CAN BE DONE TO REMEDY THE SITUATION?
This sounds like a velocity problem. If it is a flushometer you can normally adjust the flow with the set screw on the flushometer top Unless it is a 1.6 GPF then you have no adjustments other than closing the angle stop a little. If it is a ball cock like a tank and bowl you can close the angle valve ( globe valve) as this valve is designed for throttling. Good luck