I have many sewing machines and have read on here recommendations as to the size needle. One such post was about the Singer 99 using a 15x1 or Singer #2020. What do the numbers refer to? I noticed my Singer 6235 made a little "ting" sound and the needle seems to be tapping the bobbin case gently. I am using Singer brand needles but sometimes I use Organ. BTW, I have 11 machines that vary in vintage.