As "jules" said clean out the plenum chamber as often as you can to stop them getting there in the first place.
As for getting them out GOOD LUCK!!!!! Sorry no easy way. But if you remove the glovebox and put the controls to inside recirculated air, with the fan off, you can just get your hand into the blower fan blades enough to get any leaves out of it. As for in the ducting is will most likely be a dash out job... Big job.!!!!
If you can see the leaves, go to Supercheap and by one of the claw things and pull them out! On the VR/s' the front vents in the centre come off with the cluster surround so might be a little easier for you to get to them.
The other thing is that the foam degrades over time and this can cause the same type of noise!
Hope this helps, regards Guy.