Thank you for reaching out via the Bose Community. Welcome to the site.
I'm sorry to hear you have experienced whistling in one ear when activating the noise cancelling feature of your A20 headset.
There are a number of reasons why this could be occurring, including modifications to the aircraft or headset. Whistling is more generally attributed to the feedback being produced when the exterior microphones used for active noise cancelling are exposed to the earcup's speaker. A key cause for this would be due to incorrectly placed or deteriorated ear cushions (your cushions should be replaced every six months (or 350 flight hours.)
If you wish to seek replacement cushions, please head over to the Bose website in your country here, then search for A20 cushions. This video shows you how to replace the cushions.
If the cushions are intact and do not require replacement, please confirm if the issue only occurs at certain altitudes and if the issue also occurs when active noise cancelling is disabled.