I used to get that as well until a crack in the head was repaired and bought an HGasket set. Also left a thin film on the inside of the window. Common failure point is on the corner exhaust side of the fourth cylinder. I was loosing minute amounts of coolant and despite driving a lot of miles, had to add once a month if that. Gradually got worse. Upon disassembly and cleanup, saw one crack running from the exhaust manifold 2nd & 3rd cylinders to their respective exhaust valve. It's the cast iron header and iron block on aluminum head that causes the stress fractures. Common problem for vehicles of this era. Neons and Saturns heads are often irreparably damaged. Post repair no more film, no more coolant smell.