Back in the 80's, my dad spent part of the fall building heater channels for our VW Karmann Ghia. He had the heating system so well built/insulated that we were literally sweating on the coldest days. Anyway, the first few times we had the heat on, we kept smelling something burning. We couldn't put our fingers on it, but oddly we thought it was corn. That weekend, he poked around in the heater channels to find burnt bits of corn kernel.

It turns out that there was a mouse in the garage that was raiding feed corn from a bag and storing it in the VW!

 

LOL
I just had a vision of popcorn shooting out the vents

 

I have same problem but mine smells like antifreeze would you happen to know the cause?

 

Leaking heater core.