Sloshing sounds from the dashboard

Dear forum

I have for almost a year experienced sloshing sounds from behind the dashboard. I believe this is an indication of gas in the heater circuit going to the heater matrix. However, neither me nor the local garage have succeeded in resolving the problem by bleeding the system from the vent hole on the pipe to the heater matrix. I have raised the coolant reservoir, kept the engine running etc. It seems to improve for a while and then it comes back. It is typically most noticeable when the car changes direction or speed, e.g. in sharp turns. The car has an a/c, if this is relevant. Also, I had the thermostat replaced recently as the temperature was always on the low side, now it is indicating normal temp. No change in sloshing related to this replacement. My car does not have en EGR intercooler (European 2002 model).

Lately I have started to worry that my head gasket may be leaking, but I cannot see much evidence to support this:

• There is no evident loss of coolant, and the coolant looks quite clear (red)
• There is no exhaust coming from the reservoir if I take the cap off
• The oil is black with no signs of water under the filling cap.

Some other observations:

• The heater works normally.
• There is some pressure in the reservoir even when the engine is cold, don't know if this is normal.

I have scoured the internet for similar experiences, but I have not found any just like mine (focus seem to be on blocked heater core, could be what experience I guess, but my heater works as normal). Would be very pleased if someone could point me towards a diagnose and also if I should be very worried about driving the car. Replacing the head gasket seems extreme. Should I try a fluid gasket sealer? May the gas in the circuit damage the water pump impeller in the long run?

I appreciate your thoughts.

Sloshing sounds from the dashboard