Band Hero

The answer to the hint is Jesse McCartney, and his song "Beautiful Soul".

The secret notes are in any instrument on any difficulty, but they are not visible on the highway themselves. The most surefire way to get this achievement is to get 100% notes hit on the song. The achievement will unlock after completion of the song.

Jessie McCartney -- Beautiful Soul

From playing and missing three notes yet not getting the achievement, then playing again and only missing two, I am pretty sure that the notes that you must 100% are somewhere in Verse 2.

For this achievement you need to hit all the notes during a certain section of the song, it's obtainable on any instrument as each instrument has it's own section of notes for the achievement, you only need to do it on one instrument, the same thing goes for difficulty, each difficulty has it's own section of notes for the achievement and again, you only need to do it once on any difficulty to unlock the achievement.

Now the problem is that the sections are unmarked and no-one really knows for certain what or where the sections are, the best way to go for this is to try and complete the song without missing a single note, that will ensure you get the achievement, so play on a difficulty/instrument combination that you're able to full combo and you'll get the achievement.

Alternatively you could use the beginner vocals/mic fan exploit to get this and all the other achievements that are similar to this one, if you have a microphone/headset and a fan you can just leave the mic leaning against the fan while it's on and the game will register it as sound input and you'll be able full combo every song without actually doing anything.

Lastly, for this specific achievement you need to play; "Jesse McCartney - Beautiful Soul", I've linked in a video below showing you the song and a full band FC, you don't need to be playing with a full band for this achievement, but the video shows you what to expect for every instrument on it's hardest setting.

* ^{Credit to RevengeOfPazrevenge for the video}