Over time debris and other things can collect near the heater core, its possible that something small and plastic has fallen down through the defrost vents and gets hot when the vehicle is running. You may need to disassemble the heater and remove the items and perhaps vacuum it out.
a burnt rubber ball in the cardorater
Freshana AIRIA works wonders on getting rid of burnt rubber smells!!
Acrid
rubber belt broke lint overflow
probably being over used and the rubber inside is burning and melting....
Where ?on what? Mostly on a belt driving something
burning rubber
Usually that means he/she has been under a car.
sounds like your air conditioner compressor is bad.
Friction from excessive braking causes heat, thus the lubricant on brakes release this burnt smell.
A fuel leak. A coolant leak, coolant has a sweet smell. An exhaust leak. A burnt clutch. A burnt rubber smell, from a slipping accessory belt/tensioner or rubber contacting the exhaust. A Moldy smell with the A/C on from a dirty/moldy evaporator. An oil leak, if it is leaking near the exhaust manifold, from say the valve cover. A smell from burnt brakes from a faulty brake caliper. A burnt plastic smell, from plastic melting against the exhaust( wiring loom, or plastic bag on catalytic converter). A. mildew smell caused by a water leak, clogged evap case drain, flogged sunroof drain.
poop