Our Gemara on Amud Aleph tells us that Rabbi Yochanan ben Zakkai knew every branch of wisdom, including the conversation of ministering angels; the conversation of demons, and the conversation of palm trees.

So, what exactly are these branches of wisdom?

Rashi says here regarding the conversation of ministering angels; the conversation of demons, and the conversation of palm trees , “I do not know what it is.” 

However, other sages do discuss this matter. Rabbenu Bechaye Vayikra (11:13) implies them to be some kind of revelation of the future heard in the noises and rustling. One notable point Is that he transposes the conversation of ministering angels to the noises of the birds. This is odd and made me wonder if he had a different text, However, I was zocheh, thanks to sefaria’s incredible search capabilities, to find the source. It seems to be in Zohar III:228a which equates the rustling of the branches to the rustling of the wings of birds and ultimately angels as expressing divine potentialities and emanations in nature.

Agra Dekalla (Devarim 20:2) makes an interesting point. He says if birds and other creatures speak languages that could be understood, why then are humans considered distinct by Jewish tradition in that they have speech? He therefore explains that they are not really speaking as in a language, but rather their movements and their chirping are expressions of truth and divine emanations that they are picking up on, we might say, much as an antenna picks up radio waves and resonance.

Along these lines, taking into account that these creatures are able to pick up certain forces and potentialities, Rabbenu Gershom (Bava Basra 134a) and Arukh (Letter Samech) says that sages are able to predict the future by means of tuning into the emanations. 

Rashbam (Bava Basra ibid) takes a different approach and  explains that the conversations are actually ways of making various oaths that can influence the forces behind them, such as angels in charge of these forces. This Is In the manner that Kemeyas or amulets work.

The Meiri here in Succah (28a) following his rationalist and Maimonidean bent, re-interprets this as discussions in natural sciences about the trees and the heavenly forces.

But returning to Rashi, he is truly perplexing when he says I do not know what these terms are, is that aside from many other commentaries seeming to know what they are, we have his own grandson, Rashbam, offering an original interpretation. How would Rashbam know more than Rashi here? Even more perplexing, and this point is raised by ( כסא רחמים על מסכת סופרים טז:ט ), Rashi himself quotes the Arukh many times in his commentary, so we know he was familiar with it. If so, how can we understand Rashi saying over here, “I don’t know?”

The simple explanation is that Rashi didn’t really mean “I don’t know”, rather, he meant “I do not have specific knowledge, traditions and authority on this matter.” Rashi may have been particularly reticent to speak out of turn, as this material was not simply sevaros, logical analysis, but part of esoteric Torah lore. Perhaps here, Rashi felt that it was inappropriate to speak without a specific tradition from his masters. This would certainly be appropriate on the very page of Talmid that discusses the virtue of certain rabbis who would not say anything that they did not have a tradition from their teachers. Now we know Rashi from time to time publicly differs with his teachers, we see that in his commentary (see for example, Shabbos 85b, 101b, Rosh Hashana 25a, amongst many, many other places in Shas). But perhaps he felt that it was inordinately disrespectful to speak of esoterica that he did not learn specifically from his teachers, especially on the very same page where other great Rabbis abstained. If I am right about this, we see a fascinating principle of respect that Rashi employed. Even though he can at times disagree with his masters and say interpretations of his own, there are times out of respect where he specifically chose not to, based on the particulars of the situation.