Band Hero

The secret notes are somewhere in the solo. Just confirmed it because I missed one note in the entire song, and it was in the solo, and I didn't get the achievement. Then I played it again, purposely missed a note earlier on and got 100% on the solo, and I got the achievement.

The way i got this achievement was during my beginner vocals career run, i just played through the song Lips of an Angel by Hinder (check the hint) and it popped up after i beat this song. Not to sure if this concept of playing vocals applies to all Secret Notes because im missing some in my vocals run which ill go back on other instruments

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; "Hinder - Lips Of An Angel", I've linked in a video below showing you the song and an expert/guitar FC, remember though, you don't need to play on this instrument or this difficulty setting.

* ^{Credit to wolformer for the video}

Any of the secret notes in this game can be easily obtained if you can play on expert. They unlock on any instrument, and any difficulty. But your an expert player and can hit most notes, just play the game as normal and all of these should unlock.

Hinder = The Hint

Hinder - The Lips of an Angel

If you use the fan trick. (Put a mic in-front of a fan on the beginner dificulty, you'll hit every note.)

Easy Peasy.