Hey folks - sorry haven't been around.
Still haven't solved the problem, but its something to do with the air recirculate thingy.
If it's on recirculate, the noise happens. Turn recirculate off, and the fans off, then just fan back on - it's fine sometimes, but sometimes not.

Done some looking online and could be the intake door control linkage, but it's behind the glove box, so painful to get to!

Anyone know where to find the exploded diagram for this?