Thanks everyone for your help. I decided not to buy the car. I swapped the vacuum pump for the owner (I work with her) and the knocking went away. I decided, however, not to buy the car. It developed a high speed vibration, like when a drive shaft is out of balance, and the alternator would overcharge for short bursts, and actually load the engine enough to slow the car briefly. Too many headaches for me. It needs nearly $2000.00 in parts alone to fix the car. Time to continue the search.
Joe