Hiss or woosh when opening coolant expansion tank

hey guys. recently ive been checking my coolant and as i open the coolant expansion bottle, i hear a short hiss/woosh just like air coming out when opening a soda can.
I was just wondering, is it normal to hear air coming out with a brief hiss or a woosh when opening the coolant expansion tank after operating temperature?

You don’t say the year/make of the vehicle, but if there’s no radiator cap, the expansion tank has the pressure cap.

And depending on the temp of the coolant, there could still be pressure in the cooling system.

Does the cap on the expansion tank state, Do Not Open When Hot?


its a 2006 saab 9-3. it does not have a radiator cap and the expansion tank has the pressure cap. so is it normal to hear that?

Not when the engine’s cold.