What I thought was an intermittent buzz from the dashboard vent area seems to be coming from the casing behind the interior mirror, that covers the sensor. When I press it between thumb and finger it stops buzzing.

Does this come apart easily to check out? It has no visible screws and I don't want to pull on anything that isn't meant to move, especially as it is stuck to the windscreen!

Don't know about it coming part but I had the same thing and, funnily enough I had the same thing on my last Subaru which also had an auto dimming mirror, a wee bit of blue tac, rolled between mucky fingers so that it takes on a black colour, gently pressed into the seam between the 2 parts works a treat and is of course removable if necessary.

Might try that, except plastic is light grey on mine! But it isn't visible from driver's seat anyway.

I've also had it previously, on Primera GT, but that was the actual mirror glass itself, buzzing in its "frame", and also responded to securing with a bit of adhesive.

plastic is light grey on mine

Just don't get your fingers quite as mucky or alternatively roll the blu tac for less time