Valve or Fuel Injector Tapping noise

1997 Lexus ES 300 Mileage 177,000. I paid approximately $750 for a valve adjustment job approximately 2 1/2 years ago at a local Toyota dealership due to tapping noise. Upon completion of maintenance procedures, engine continued to make tapping noise. Two weeks ago, I took my vehicle to an authorized Lexus/Toyota repair shop and the technician informed me that my fuel injectors were causing this annoying tapping noise. The technician asked me to place my hand on the end of the fuel rail and I felt a vibration with the engine running.
Is it possible for the injectors to make a tapping noise? If injectors are defective should I attempt to repair with over the counter injector cleaners first or have technician replace defective injectors? Is it safe to purchase used injectors (from Ebay) or buy new injectors?