Strange constant fan/motor sound

[HPsauce]

Not sure if this is engine bay or inside (heater/aircon) but you can hear it inside the car. It's annoying, worrying and perplexing.
Bonnet open and it appears to be central just in front of the windscreen under the big air intake.

It's a steady noise, but only there when the engine is running, does NOT vary in speed at all and nothing I do changes it, other than turning the engine off.
Turning the ventilation off makes no difference (it stays there) and it's not on if you turn the key, until the engine has actually started.

I'm running the ventilation in "econ" mode (no aircon) anyway until I get round to clearing the "orange" drains; at first I thought that might be related, but changing anything & everything on the ventilation makes no difference at all.

It sounds like a fan with a "slightly dodgy" bearing or with some dirt caught in it. Whether it started today or just got a bit louder I can't really say.
At speeds above about 40/50mph it's not noticeable (depending on the road surface noise).

Any thoughts anyone?