Old upright pianos tend to develop all sorts of strange clunks, clicks, and buzzes. One cause of these noises is worn hammer rail felt. Over time, this felt becomes compacted and no longer provides adequate cushion for the hammer shanks. The compacted felt gives a dull thud when the shanks return to rest. Fortunately, this felt is very easy and quick to replace.
Begin by removing the hammer rail from the piano action. It will have 3 or 4 metal prongs that are inserted into bushed holes in the action brackets. One prong will face the opposite direction of the others. Bend this prong back enough to clear the action bracket, and lift the hammer rail away from the action.
On some pianos, it is easier to just remove the action bracket that the odd-ball prong fits into, rather than bending the prong.
Once the hammer rail is removed, peel away the old rail cloth and measure its width. Buy a roll of new hammer rail felt of the same width.
Purchase a bottle of wallpaper remover and mix a small amount with water according to the instructions. Use an old toothbrush to wet the old glue with the remover. After a few minutes, the old glue should easily scrape off.
Apply PVC-E or hide glue to the lower half of the hammer rail and lay the new felt across it.
It is important to not glue the top edge of the felt. Leaving this edge loose allows a bit more cushion for the falling hammers
After the glue dries, trim the ends of the felt flush with the ends of the hammer rail.
Reinstall the rail onto the piano action and either bend the prong back into place or reinstall the action bracket that was removed.