Top: time-distance maps for the moving (left panel) and steady (right panel) slit (red lines in Fig. 1c). The time distance map of the top left panel is included in the on-line movie; it shows the loop evolution. Middle: zoomed parts of the time-distance maps: the red and blue bullets are the maxima and minima positions. Bottom: period vs. time as inferred from the intensity profiles above (red lines in the time-distance maps). The vertical dot-dashed lines show the different oscillation regimes. The dashed line shows the linear trend in the period, with the initial value about 214 s and the increment of 0.43 s/min (left panel), and the average period of 224.19 s (right panel).