Jun 16, 2019
Thanks for reaching out and welcome to the Bose Community Forums.
I am sorry that you were having issues with the 2.5.5 firmware upgrade. We have since released a new software version, please can you follow the steps below to update your headphones and then reset them and let me know if you are still having issues after that.
I look forward to hearing from you!
Jul 11, 2019
I had run into the same issue where ANC noticeably went bad after the firmware upgrade by the Bose Connect App. Browsing for the solution online, I stumbled upon this very useful article which solved my issue https://www.androidauthority.com/bose-quietcomfort-35-ii-anc-firmware-1010335/. Indeed the issue was the broken firmware update by the Bose Connect App and updating it from Bose Updater on my Mac fixed the issue. Poor hissing sound is now gone and I'm back to the smooth ANC that I had expected of and experienced on my QC 35 II out of the box. Happy Boser.