Context. The nature of loops is still a matter of debate with several explanations having been put forward. Simultaneous spectral and imaging data have the capacity to provide a new insight into mass motions, dynamics and energetics of loops. Aims. We report on spectral data taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) and imaging data from the Transition Region and Coronal Explorer (TRACE) of a transient event which occurred in a cold loop, lasting a few minutes. Methods. A sequence of TRACE images in the 1550 Å and 171 Å filters show a disturbance which originated at one foot-point and propagates along the loop. The SUMER slit was placed at the other foot-point of the loop. In order to interpret the results, numerical simulations were performed with the results then converted into observable quantities and compared with the data. Results. During the event a radiance increase and a relative red shift of ≈km s-1as detected in the N V 1238.82 Å line. 1D numerical simulations are performed and observable quantities derived from the results of the simulations. The observed dynamic behaviour of the N V 1238.82 A line profiles was recovered. Conclusions. The results suggest that the observations could be interpreted in terms of a short-lived siphon flow reaching a speed of 120 km s-1 and driven by a nonlinear heating pulse. The energies required to drive the observed red-shifts are estimated to be about 1025 erg. The absence of a significant blue-shift caused by the return flow is explained.