The knot tied in the Belt makes a "thump" noise when it passes theough the Pulley, which can clearly be heard during quieter passages of music. I did read read a long while ago someone else had the same issue, but they were uncertain what it was, well if that person is reading this, this is the cause. Anyway else had the issue and found a way of rectifying this?
By the way a "better sounding" belt is replacing the "fishing line" with Radio Dial Cord, substantial improvement. Both types make the "thump" noise when the knot passes through the Pulley.