How many heat cycles has it been through, the return from the turbo is where the air should be seen as the system bleeds itself, very effectively as well, this is through here being the highest point, the water gets hotter after switching off as it absorbs the heat in the engine but without being circulated just gets hotter and hotter, after a few 2-3 heat cycles shouldn't have any air left if there though.
Temporarily bypass the water circuit on the turbo and see if the bubbling stops.
tabetha