Question of etiquette
Would it be rude to leave a note for the upstairs neighbors, asking that if they continue to have regular vigorous sex, that they at least put some padding between their bed and the wall?
They presumably do not notice that the entire wall is vibrating, down at least as far as my floor.
They presumably do not notice that the entire wall is vibrating, down at least as far as my floor.

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Your wording is a good start, though. I might even go more euphemistic and just say there seemed to be something hitting the wall.
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"Hello, I'm your downstairs neighbor, and I wanted to let you know that I've recently been awakened/kept awake/seriously distracted by wall-thumping sounds from your apartment. I'm sure you haven't realized that the sound carries through the downstairs wall, and I hate to bring up this awkward issue, but I hope you might find a way to minimize the noise--perhaps some rearranging of furniture?"
...ack, this is harder than Miss Manners makes it look.
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on the other hand
Re: on the other hand