It started today, and it stops when you push in the "air recirculation"
button. Happens with engine off and ignition on. I gather from the net
the problem is the air control flap gearbox has stripped. But how do you
get to it?

From under the bonnet and immediately under the windscreen I can get to
the intake and remove the pollen filter (very dirty) and see down the
intake past the flap to the (constantly?) turning fan. But how do I get
to the stripped gear?

There are nuts either side of the intake, but do these drop the whole
fan unit inside? Car is a 95 turbo diesel with aircon ... which works
but puts a drag on the engine.

Thanks for all assistance.