The analysis of a sentence #1

The narrator recalls his childhood.
He has lived with his mother, Peggotty the only maid of his house, his stern stepfather Mr. Murdstone, and Mr. Mudstone's eccentric elder sister in his late father's house in Blunderstone.
He was going to a boarding school near London on a cart, arrived the way point Yarmouth.
He now resumes his traveling on a new wagon with some other travelers.

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The night was not so pleasant as the evening, for it got chilly, and being put between two gentlemen (the rough-faced one and another) to prevent my tumbling off the coach, I was nearly smothered by their falling asleep, and completely blocking me up.
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This lady had a basket with her, and she hadn't known what to do with it for a long time, until she found that, on account of my legs being short, it could go underneath me. It cramped and hurt me so, that it made me perfectly miserable, but if I moved in the least, and made a glass that was in the basket rattle against something else (as it was sure to do), she gave me the cruellest poke with her foot, and said, "Come, don't you fidget. Your bones are young enough I'm sure!"
At last the sun rose, and then my companions seemed to sleep easier. The difficulties under which they had laboured all night, and which had found utterance in the most terrific gasps ans snorts, are not to be conceived.
[David Copperfield by Charles Dickens]
I think "which had found utterance" modifies "the difficulties."
If so, I was wondering what "the difficulties which had found utterance" means.
Thank you in advance for your help.