Hi Pietro @p.alfarano
That metallic sound can come from several sources. Most frequently these are: 1) the reed is not parallel to its slot; 2) the reed is twisting during the bend.
To see if the first problem applies, you must take the harp apart and remove the reedplates from the comb. Hold the reedplate up towards the sky or a brightly lit wall and look at the offending reed. There should be an even space along each long side of the reed for the light to pass between the reed slot and the long sides of the reed. If that is not what you see then you need to attempt to slightly and very carefully pivot the reed at its rivet so that this space becomes correct (= even along each long side of the reed).
The solution just described (to pivot the reed at its rivet) is most easily and safely done using a harmonica repair kit reed wrench that can be purchased online from Hohner or Seydel or some individual sellers.
If the first problem is not what you find, then the it is most likely the second: the reed is twisting along its long axis when you are attempting a bend. This problem has occurred frequently (actually on all) of the Suzuki Pro master harps I own and on one Suzuki Blues Master. It is not easy to repair but if that is your problem, let me know and I will attempt to explain how it can be approached.
Regards,
– Slim 