It is possibly play in the wiper linkage as this only makes noise when the wipers are on the glass this is due to the fact that when the wipers are on the glass and moving there is load on the motor/ linkage due to friction from the window and spring tension from the arms. To confirm any play / movement with the wipers off try and move the arms back and forth to notice any excess free play. There could even be something contacting the linkage to make this noise in a certain location. Try and lubricte all wiper linkage pivot points under cowl panel and don't forget the wiper arm posts bottom side, this may help the noise situation . With your wipers on and runnig and you outside the car it should not be to hard to identify where the noise is coming from.
