In the Montserrat Massif (Catalonia, NE of Spain) rockfall risk arises, since the significant hazard of the mountain is combined with the number of visitors attracted both by the cultural and touristic spot of the monastery area, and by the natural spot for hikers and climbers. To address the risk, a mitigation plan is underway, including rockfall monitoring at different scales and using different techniques. This presentation focuses on the case of the rock block A3-6 on the top of the cliff placed between the road and railway leading to the monastery. This block has been monitored since 2010 by extensometers that show the cyclic movement according to annual thermal cycle, which is mainly recoverable, but small residual plastic derivations have been detected in a varying amount, along years. At the end of 2021, a stabilization work has been carried out. Just before, during, and after the stabilization works, the block has been monitored by passive seismic techniques to detect signs of the evolving stability similarly to previous experiences in the Alps. Two main results have been found. On the one hand, a slight difference in the recorded ambient noise was detected between the potentially unstable block and the rear massif. In the range of 10 to 40 Hz both the H/V quotient and the polar spectrogram differ in amplitude. A characteristic resonance frequency of the block is observed around 20 Hz in the direction of the toppling instability mechanism. On the other hand, during the drilling works of the anchor bolts, it was possible to clearly detect the moment when the drilling hammer crosses the rear joint of the block. This allows a confirmation of the assumed geometry of the block and the required anchor length, as well as a qualitative assessment of the persistence of the joint and its mechanical contact.