This will be a long post, scroll down to the last paragraph for the conclusion.
After much research and after reading numerous positive reviews, I decided to buy the QC35 II - the reigning champ of active noice cancellation according to most sources. My expectations were high to say the least, and for the most part, the headphones lived up to the expectations. They look great, they are extremely comfortable, the sound is balanced and of high quality and the ANC is very effective. However...
When the ANC function is active (high or low, doesn't matter) a loud whitenoise is introduced. This hiss drowns any sound unless the sound is turned up to more than a quarter of the maximum volume. No matter the volume, whenever there is a pause in the audio, the hiss becomes apparent. Obviously I thought that something must be wrong. My dad owns a pair of QC25s, so I tried them out. They're dead silent. My other basis of comparison is a pair of Sennheiser Momentum II, which I currently also own. Granted, the Bose ANC is much more effective at cancelling external noise, but the Sennheiser ones don't introduce any whitenoise what so ever.
I reached out to my local Bose costumer support and described my problem. The lady I talked to also thought that this sounded like a faulty product and managed an exchange. Props to the costumer support, I'm very pleased with the support I got. Two weeks later, I receive a new pair of headphones. With the exact same problem. The chance of receiving two products, from two different batches (confirmed by the manufacturing date provided by the app) with the exact same quality fault is miniscule. I have therefore, in combination with multiple hours of research and problem solving on my own, come to the conclusion that this is just the way it is.
Unless the environment is extremely noisy (e.g. you're on an airplane or in a 1998 Renault with studded tires on the motorway), it's no use to activate the ANC, since the noise it introduces is more annoying than the environmental noises it cancells. Considering the ANC is the main selling point of the QC35 II, I am very disappointed. Now, am I particularly sensitive to this noise, more so than the average Joe? Maybe. Maybe not. Lesson to be learnt: Try them on IN A QUIET ENVIRONMENT before deciding to buy. I didn't.
Thanks for posting. We appreciate you sharing your experience with your headphones. ANC effects people different ways and a lot of what you are experiencing could be perception. Thank you for your feedback and want you to know that if you need anything else don't hesitate to reach out here.