My vehicle heater fan does not work.

My vehicle heater fan does not work.
When, in my Fiat 280 from 1986, I close the outside air valve (upper left slider to the right) and turn on the fan (bottom right slider to the right), then I hear a lot of noise, as if the fan is running, but the heater is not blowing. So it does not get warm inside and the windows herds remain in the cabin. There is only a very weak warm air stream. Does anyone of you know how to fix this?

This turned out to be a very expensive repare at a garage.1 I paided about € 1.000,- for a new heater and heater box. Hope it will keep working...