Since you smell it outside sometimes, my guess is that it is not the HVAC and it’s something under the hood. The HVAC draws air from just below the windshield, so if you have some fumes coming up from under the hood, they can get sucked into the cabin when the HVAC is on, but the smell will be reduced when the HVAC is off. If the problem was in the HVAC itself, I don’t think there is any way you’d smell it outside the car.
You mention that you checked the manifolds and don’t see anything. I’m not sure how you are checking, they appear to be completely shielded on my car - I can’t see any part of the exhaust manifold itself, and its pretty tough to get a look all along that lower edge of the valve cover as well. Might be able to see with a borescope. Valve cover leaks seem to be pretty common in these cars, and generally don’t leak enough to change the oil level (just a seep) until they get pretty bad.
The other thing that occurs to me is a slipping belt, which can cause a burning smell.
The slipping belt is interesting, next time I smell it I'll see if I can pinpoint the location better or determine if it's a rubbery smell. I was poking around looking at the exhaust and everything when I had it up on the lift (benefit of having access to the public works garage!) and I did the best I could to look for leaks or signs of burning/burnt oil.
I’ve smelled burning PS fluid or Transmission fluid (hydraulic type smell) once or twice, usually on long interstate trips once I get off on an exit. Transmission states it will vent excess fluid and it happens to drip on the exhaust.
I'll check that a bit closer next time I get underneath it, the exhaust pipes all looked clean (minus the occasional small spot of rust)