What causes the sound of climaxing bees?

I just saw the funniest video of bumblebees(?) having sex. (mirror) As they were getting it on, there seemed to be an almost audible ‘moaning’ coming from the pair. From what I presume it was a sexual climax we heard!

Bees have neither vocal chords nor lungs, so I wonder: what is that sound?

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As the pair is mating on a newspaper, I’m almost certain we’re hearing transduced vibrations from the bees made audible through the paper and not the bees themselves. (Nonetheless, what a beautifully humorous anthropomorphism! Moaning bees certainly give vivid color to the birds and the bees!)

I have multiple theories:

1. The ejaculation of sperm by the drone is known to be ‘explosive’, even rupturing the endophallus and making a ‘popping’ sound. I wouldn’t call the sound in the video ‘popping’ by any means, though.
2. (Un)controlled movements of legs or wings by either of the pair. This seems very likely considering the subjective frequency of the sound, but that’s just speculation on my part.
3. Perhaps it’s the queen piping? I’m really grasping with this one. I don’t even know if that species’ queens pipe or even have the ability to pipe.

Any excited entomologists wanna fill me in?