I replaced the coolant tank yesterday, got one from Volvo. I noticed that even when the engine is cold, there is still pressure in the system. When you remove the cap, you can hear hissing, and the level of coolant rises back up in the tank. I couldn't see this happening with the old one, due to the fact is was almost opaque from age and other stuff. Is this normal behavior? I have since added a bit more coolant to the res. but this makes me a bit nervous....maybe exhaust gases trapped in the coolant? Or am I just paranoid? Advice?