The Wood Block is sometimes, but rarely, used in orchestral scores. It has MIDI instrument number 115.
The castanets, archetypically employed by Spanish flamenco dancers, can be synthesised using MIDI, but only in GM2, XG or GS, as GM does not have such an instrument. It is found as a bank select variation on GM 115, the woodblock. When using GS, set LSB = 0 and MSB = 8.. When using GM2, set LSB = 1 and MSB = 0. When using XG, set LSB = 96 and MSB = 0.