Commentary Par XXI 136-142

Peter's collegial souls, as they descend the ladder to greet him, glow with righteous indignation at these words.  Surrounding him, having ceased their circular movement, they let loose a cry so loud and angry that Dante cannot make out the words of what they shout.  Where earlier in the canto he had been denied both Beatrice's smile and the singing of the blessed, now he is allowed to hear a superfluity of sound with a similar net result.  (The canto moves from monastic silence to monkish outrage, each of them leaving the protagonist stunned, uncomprehending.)  It is a final reminder of his human incapacity even now, when he has attained the height of Saturn in the heavens.

For this 'thunder' as resonating with that found in Ovid's description of Jupiter, preparing to descend to seduce Semele by taking his thunder and his lightning bolts along (Ovid, Metam. III.300), see Brownlee (Brow.1991.2), pp. 226-27.