Air in heater matrix
 

Hi I have a 2004 wrx and I had air in matrix and running water sound when starting from cold I bleed the system it's been fine for a week and now it's back again is this an indication I have the beginning of a head gasket failure? I'm not losing any coolant in rad or header ant the expansion bottle has stayed the same level throughout fingers crossed I didn't get all the air out last time I bled it

The temp gauge goes to just under half way and I have no loss of performance any ideas?? Many thanks jody