We generate frequency-tunable narrow-band intense fields in the terahertz (THz) range by optical rectification of a temporally modulated near-infrared laser pumping a nonlinear organic crystal. Carrier-frequency tunability between 0.5 and 6.5THz is achieved by changing the modulation period of the laser pump. This tunable narrow-band THz source allows the selective coherent excitation of adjacent vibrational modes, which are demonstrated for two phonons with a frequency offset of 0.8THz in single-crystal SrCu2(BO3)(2). Our compact and scalable source enables an effective approach for the advanced manipulation of low-energy collective modes in condensed matter and has the potential to reveal the coupling of specific lattice vibrations with other degrees of freedom.