I would say probably the resistor
I just did the same thing, and had the same symptoms before it finally stopped working it was like 33 bucks at orielly and now my heater works great. All the speeds work.
The hissing sound when you change the zone controls is bc the zone controls are pneumatic.
Oh and on that resistor one of the back screws is like impossible to get out. I ended up getting some long needle nose pliers and ripping the old one out breaking off a corner.(hey if its broke how much more bad off can i get) and then taking it out.
I know i should have tried harder to but i just didnt put that one back in.
