Unwanted string-like sounds when playing

When playing a piano score with the SFX soundfont "SalamanderGrandPianoV3", there are after only very few seconds, additional sounds, just like if I had another track with violins.
This issue disappears when I use the original soundfont FluidR3Mono_GM.
This effect is not random : the same sounds are played at the same moment...
Those sounds are also present in the output files (here the .ogg)