coolant bubbling cup 172

[renaultsport1889]

I lifted and bonnet and noticed the coolant bubbling. Temperature gauge always shows half way, so never over heats. Haven't noticed any coolant loss in months, or loss of power. The oil cap is fine, no milky residue

Is it normal for coolant to bubble evry now and then.

I take it theres nothing wrong with head gasket?

SOme advice woould be great