Hi, I am looking for some advice. Recently my Fiesta MK6 had to have a new radiator, head gasket, thermostat replaced due to an overheating problem.

Following this, the internal heater only blows cool air, I've heard a few rattling sounds coming from the dashboard. Would this blowing cool air also cause some overheating? Since the work when stuck in traffic after a while driving (30 minutes or so) i have noticed the temperature gauge spike to above normal (not critical) and the radiator fan kick in. Once i get to drive in clear air the gauge decreases back to normal. The coolant levels are still the same and i have not needed to top up so far.

your radiator fan is doing exactly what its supposed to do dont worry about that, if your levels are fine then your car cooling system is fine too. with regards to your car only blowing cool air, it will be something to do with your heater matrix, maybe 100 or so to fix, give it a few weeks, heat isnt really important in the summer

First thing is to make sure the heater control valve is opening and the coolant levels aren't low. Other possibility is that the heater matrix is air locked and needs bleeding.