Hi Boyd,
This is a pretty vague description of your problem, but here goes. If it strikes on a coiled gong, the fixing screws/nuts may need tightening. If the hammer has lost its leather on the end and now the metal is striking the gong, the leather insert should be replaced. This can be done by removing the hammer and replacing the leather insert, sometimes just getting a suitable size leather lace and cutting off a bit and gluing it carefully into the hammer. You can also buy a new hammer from clock suppliers, Meadows and Passmore, Brighton. If this does not help, I suggest your local clock repairer may be able to help.
Good luck,
Richard, November 2007