you just released the pressure in the system by opening the header tank, youre not meant to open it while the engine and water is still hot, theres nothing wrong with the bubling as you mentioned. Unless theres oil in the water. head gasket failure.
but yours dont sound like it is.
on the dash, where does the needle sit? on the 'norm' writing is fine.

have you fitted a fan over-rider switch. what mods have you done?

is your termostat fine? whats your temp sender unit like? you done a water and coolant change lately? your fan does kick in at the correct temp? does it still work?