Coolant reservoir rumbling (2007 Mazda5)

[hleboklop]

Hello,

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?

 

[negusm]

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.

 

[hleboklop]

And it seems like it was a bad radiator cap and cracked filler neck!