Coolant reservoir rumbling (2007 Mazda5)


I'm trying to figure out, why does the overflow reservoir on 2007 Mazda 5 sounds like that.
Could it be a leak in reservoir?
Faulty radiator cap stuck open?
Blown head gasket? (I haven't seen any foam or choco smoothie.. yet)
Have you ever experienced anything like this?


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Not overheating? I would suspect a blown head gasket overheating the coolant and it bubbling into the overflow. But if the car isn't overheating...I dunno what that is other than a bad radiator cap.