If the heating system is not sending too much heat at low speed and you still hear noise, then your fan is likely to be screwed up. You have now entered, the fan is only used at low speed and in neutral. Put a little WD 40 to lubricate and voila. Activate the mechanisms to see where it is blocking.

To remedy this it is simple, remove the cover and look inside how it was done. These shutters, which allow you to direct the air or adjust the intensity, sometimes get blocked over time. It is a concern that happens quite regularly. Air shutters blocked on the Opel Insignia

The only concern is that you won't be able to really control the air temperature. Because the natural heat of the engine will increase little by little, so will the cooling circuit, whether the thermostat works or not. Even if the thermostat of this Opel Insignia no longer works properly, the heating will still work. The concept is simple, in the case where the thermostat which regulates the temperature does not understand well the temperature of the water in the circuit, the latter is affected.

Recurring heating problems on an Opel Insignia Broken thermostat