This is not the definitive answer!
This is just a summary of the information we got about his last video, a list of failed attempts and potentially relevant elements for new approaches.
Elements contained in the video:
- The name of the video: 0101 0101 0000 0111
- Usual intro music
- Suite No. 3 in D major by Johann Sebastian Bach, also known as "Air on the G string"
- The string "ISL 4 $ $ $ $ 3 xEFTvZ"
Analysis of these elements:
The name of the video is, as usually, a binary number. Title numbers are always consecutive, in this case 0101 0101 0000 0111, which means 5507 in decimal. There seems to be no hint in the title.
As well, we have the typical intro music, basically NOC's signature. No info here!
Then we listen "Suite No. 3 in D major". What may be useful in it? Well, we have the number 3 and the capital letter D, but those information appear to be pretty irrelevant.
The real information is likely to be in the second name of that music, which is "Air on the G string". The words "Air" and "string" have the same length of "ISL" and "xEFTvZ", suggesting a direct correspondence between them.
Also, the word "On" could be interpreted as "1" (ON=1, OFF=0), so both strings can be perfectly aligned now. What about the "G"? When written in lower-case, "g" tends to be visually similar to the number "9", even though this appears captious.
Well, know we have the nucleus of the puzzle, the string ISL 4 $ $ $ $ 3 xEFTvZ.
The most notably features of this string are the presence of exactly 11 alpha-numeric characters, as in all Youtube videos ID (a video ID is what we actually expect to find), the sentence "IS LEFT Z" formed by the capital letters, an insolit separation between "4" and "3" (my ruler shows that it's made of 3 blank-spaces). As suggested before, these words align pretty well with the title of the music.
Someone also pointed out that exactly 11 notes are played in the music. This might be a great hint, if confirmed!
It's almost obvious that we have to deal with a cipher, as we already did earlier. A Caesar's cipher would be too trivial, we've already solved that kind of cipher in a previous video.
Vigenere cipher is the top candidate to solve this mystery, since we are dealing with two strings of similar length.
Do we have to add or subtract the strings? We don't really know, try both!
"ISL 4 $ $ $ $ 3 xEFTvZ"
Potentially useful keys:
AIR 1 G STRING
AIR 1 9 STRING
AIR ? ? STRING where ? means any number/letter between 0-9/A-Z. This generalizes the AIR STRING approach.
Also, the 11 notes in the video could be a key, if only we could find their names.
What if nothing of this works?
Well, we would definitely need a new idea! We also might need to dig into his 3 previous public videos, looking for additional hints.
What can I do to help?
First, you have to officially verify that Caesar's cipher doesn't work at all. It requires only 26 attempts, if you do it please confirm its non-validity in the comments.
Then, hoping that you have a discrete experience with programming and coding (especially PHP) you could brute-force all the Vigenere keys suggested above and check if that video exists. Please be accurate, a minor mistake in your code is likely to miss the ID we're looking for!
Also, if you have time to waste (or your pc has), apply Caesar to all the ID you got and verify them as well!
If you're experienced with music, please verify that the played notes are 11. If you are able, identify them!
Sources and credits:
My reason, comments below the video, this discussion. Thanks to Ian MacDonald for noticing the 11 notes.