Go to practice mod
Select the song
Choose the first section of the song (intro or whatever)
Choose slowest difficulty  - you don't have to hit all the notes
Plug out your controller right before the percentage screen is about to come up
If the screen keeps rolling without notes than that's good
Then exit and go to career and select that same song

There will be no noise and it will look like you are in practice mode
Then just get 100% on the intro or first section, and the game will do the rest
WARNING - after you hit all the notes and the game does the rest, don't strum or hit random notes because then you will not get 100%