The heater on my ford focus makes a wailing sound when running?

The heater on my ford focus makes a wailing sound when running and only works on full power. When it's wailing the airflow to the car is really restricted, but sometimes it works fine - when it works properly you could turn it off then on again and it will start wailing again and the airflow is rubbish.

Any ideas?

The wailing sounds like it's coming from behind the glovebox on the left somewhere.