My Sierra 2.0 DOHC looses coolant with no visible signs of a leak. I am 99% sure it isnt the head gasket because its not pressurising and isnt mixing oil and water etc. When the car is driving its fine and doesnt loose any coolant but when it is sitting in traffic etc it starts leaking somewhere. i also noticed today that it started dripping underneath the car beside the gearbox running down the tunnel. Any help would be greatly appreciated, and if anything is unclear ask me.