Many thanks all for the informed replies.
After speaking with Bose support and trying on a friends headset, hearing a very similar level of static, I’ve decided not to return mine.
It seems I am simply a little more susceptible to high frequency sounds than average - probably unsurprising given I’m in my early twenties and a newbie who has not yet been exposed to lots of constant loud noise. This was further confirmed by having my parents (late 50s) try on the headset, only for one to find the static to be barely perceptible and the other not able to hear it at all!
The headset seems to be doing its job perfectly well when being used in its intended setting - close proximity to an aircraft engine, rather than a quiet room. So I’m happy!