Twitter/X showed me only the first post but here is the whole thread:

https://threadreaderapp.com/thread/1874853029872365961.html

Still no references, only a link to a sound playlist of elephant noises called "ElephantVoices":

https://soundcloud.com/elephantvoices

I found a brief mention related to this from 2017:

> Dr Joyce Poole and her colleagues noticed one of the younger African savannah elephants making truck-like sounds that were unlike any of the calls they had heard from African elephants in the past. They realised that from her night compound, the elephant could hear the trucks three kilometres away on the highway.

That Dr. Poole seems to be the person behind ElephantVoices as reported by National Geographic:

https://www.nationalgeographic.com/animals/article/elephants...

Update: ElephantVoices has an explanation of the playlist in their website under "The elephant ethnogram":

https://www.elephantvoices.org/elephant-ethogram.html