I have an old farm house (built 1923) that in on well water... the water is pumped into an old single chamber water pressure tank. When the well pump turns off, the pipes that lead into the pressure vessel knock loudly (I assume this is water hammer)... It used to be quiet but has now gotten very loud... Do I add additional air to the pressure vessel?? Thanks for your help!

You only add air to a bladder type tank. Is there a valve on top of the tank for adding air?
If the knocking occurs only when the pump shuts off and assuming a surface type pump, the reason would be motor torque. Make sure that the pump and motor are securely anchored down.