I had switched over to the composite graphite racket [also racquet] from the olden days' wood racket. Graphite rackets are tightly strung and while the ball is struck, it produces a vibration like a string instrument! It is in a way disconcerting and hence not desired by players. To absorb this vibration, a rubber piece is attached off the centre of the strings. They were available in sports shops. But why I not experiment by making one myself? I had in the junk box some rubber stoppers of old injection bottles. I stuck them after cutting the portions that fitted the neck of the bottles, leaving just enough relief so that it takes the string in the groove formed - see picture above. It worked fine but I bought a ready made from the sports store later. The green one above is this while the one I made is on the right. Both work well.