Take the cap off.
I recently completed my Hogwarts locomotive upgrade, I used the CC-Lite and the small steam sound board, the high pitched whistle sounds like the right tone for this locomotive.
I also put a PTC in the motor leads to protect against a stalled motor as I had a CC-Lie go up in smoke when the Interurban set it was installed in hung up on a switch and apparently stalled the motor. I use a .65A hold, 1.3A trip PTC, and it's been able to pull four passenger cars around the club layout, including grades, and never had a problem. Hopefully, the PTC will prevent a repeat of my issue with the other installation.