i have a 2007 v36 sedan low mileage 64000 km
Out of the blue i heard air bubbles behind the dashboard after starting up,i lift the hood and see the coolant tank rather full,so immediatly you think head gasket,well i did.And with cap off i see big bubbles coming up not the fine ones i was expecting.
So off to the mechanic he pressure tests the cooling system no problem found so he suggests throttle body off for a leak down test and compession test on all these test nothing found to suggest head gaskets.
where can these bubbles come from? its got him a bit stumped He did say its a possibility for the water pump to suck air in,anyone seen this?
I have no water in oil and no oil in water,the car runs fine.

Thank you for taking the time to read any ideas would be great........phil