Is the smell oil or coolant? Heater matrix contains enigne coolant, which flows round the cooling system. Is the coolant level ok under the bonnet in the reservoir? Low level would be a sign of a leak.
I sometimes get an oil smell. Got an engine oil leak, and if the wind catch it correcty, the air intake grabs hold of it and shoves it in the cabin. As expected, won't do it when recirculate is selected.