Sometimes the knocking sound you hear on a grand piano as it is being played can be caused by worn out damper stop rail felt. If this is the case, it can be changed without taking the dampers off.

This job takes a lot of patience. First you must get the old felt off. Adjust the stop rail to its highest point and tighten it. Chisel the old stop rail felt off with a thin chisel or a razor blade that is glued to a thin stick. Work little by little between each damper. Vacuum out all the excess felt. Measure the length and width of each stop rail, and cut the new felt accordingly. Spread the glue on one side of the felt and let it get absorbed. Apply more glue if necessary. Cut a strip of newspaper and snake it through, under the stop rail, so if glue drops, it will fall on the paper.

Carefully snake the felt through until it is in the correct position. You can clamp it down by propping up the damper levers with a screwdriver handle and adjusting the stop rail far enough down so the felt presses on the top of the levers. Don't forget to take the newspaper out when the glue dries.