"They (the winds) angrily roar around the bolts with the great murmur of a mountain."
In lines 1-7, Vergil, without mentioning Aeneas by name, introduced us to the theme of the hero in exile, punished on land and sea by an enraged goddess. He then invoked the Muse to help him understand the reasons for Juno's anger, and in the process praised the 'pietas' (1.10) of the still unamed hero. Lines 12-33 provided us with the reasons for her anger, with much of the focus on the city of Carthage, which looms larger over Roman history as well as Vergil's Aeneid.
At line 34-35, Vergil finally directs our attention to the Trojans, but still without mention of Aeneas by name. There we learn they are sailing happily and energetically away from Sicily, and that they have just barely passed beyond the visual field of Sicily.
Vix e conspectu Siculae telluris in altum/vela dabant laeti et spumas salis aere ruebant.
But before hearing anything else about the Trojans, we are taken back up to Juno and her anger, as we observe her speaking to herself about the insults she has suffered, and traveling to Aeolia, where she seeks out Aeolus and the winds under his control. Vergil, before describing the conversation between Juno and Aeolus, describes the winds in this way:
Illi indignantes magno cum murmure montis/circum claustra fremunt
The winds are bolted within the cavern, they are resentful, and they are roaring and rumbling in their eagerness to break free from Aeolus' control. The power of the winds that will soon attack the Trojans, who are happy and just beyond sight of Sicily, is more than the destructive power to which all sailors are vulnerable. These winds are caught up in a complex political, even sexual, dynamic involving gods, lesser deities, humans, and the natural world. The winds are personified, but naturalized as well. They are resentful (like people) but rumble and roar (like winds). They are in a natural space (a cavern) but bolted in, like prisoners. When they do attack the Trojans, their power will be natural, supernatural, and symbolic of the historical, military, religious, philosophic, and sexual forces which Aeneas must negotiate throughout the entire poem.
This sentence provides us with a great example of Vergil's poetic style. 'montis' meaning 'mountain' refers both to the physical location of the cavern and, metaphorically, to the size of their rumbling. Grammatically, it is in the genitive case (translate: 'of a/the mountain) and if you had to come up with a nice, literal translation, you would translate it after 'claustra' in the next line (around the bolts of the mountain). But given its proximity to 'murmure,' its position at the end of the line, and the alliterative connection to the previous prepostional phrase (note the repeated 'm' sounds in 'magno cum murmure montis'), it is hard not to take it also with 'murmure' and translate it 'with a great, mountainous murmur' - in other words it is used metaphorically to describe the size of their rumbling.
Finally, notice how this line demonstrates that Vergil's Latin, in spite of its distance in time and space from us, is so closely linked to English: indignantes (indignant), magno (magnify etc.), cum (= con, cf. convene, etc.) murmure (murmur), montis (Montana, etc) circum (circumnavigate, etc.), claustra (claustrophobia, closet, close), and fremunt (which is related to the Greek word from which brontosaurus derives).