I have done a search and nothing seems to match, so here goes. '97 SE with 90k miles. Had a coolant flush at 75k, dealer put Dexcool in. Last week, I had the coolant by-pass elbow on the intake replaced because it was leaking. For a few months I've gotten a wiff of coolant ONLY with the defrost on. The windshield doesn't fog up, isn't greasy, and the carpet isn't wet. There are no leaks under the car. Pressure tested ok. Is it time for a manifold gasket job? Should I do the dye in the coolant test or just leave it.
Thanks

Try taking a look at your EVAP and clean it. https://www.gmforum.com/t278997/

Can you smell any coolant under the hood while the engine is running? There is a possibility coolant is dripping on something hot and your catching a whiff of it. Look around the water pump and heater lines real close.

No smell under the hood, just when the defrost is on. If I switch to floor or dash vents, smell goes away. Will check EVAP though.

I had a similar problem w/'92 Bonne SE. I noticed the coolant resevoir going down every day it was being driven. Even though no wet carpet or coolant on the ground, i had the heater core replaced. I see that you posted 'Pressure tested ok' which should have caught it. If your losing a noticable amount of coolant and can't find anywhere else, you might still consider the heater core. Good luck.