An important effect of the frequency chirp of the optical transmitter in radio over multimode fiber links is put into evidence experimentally and modeled theoretically for the first time, to our knowledge. This effect can have an important impact in short-range connections, where, although intermodal dispersion does not generally cause unacceptable limitations to the transmittable bandwidth, the presence of modal noise must be accurately kept under control, since it determines undesired real-time fluctuations of the link.