Diagnosis
Hair fragments and product residue are carbonizing on the heating element and chamber walls. This is a fire hazard.
How to Fix It
Unplug and let it cool for an hour. Use compressed air (can of keyboard duster) to blow debris out of the chamber. Use a dry, stiff-bristled brush (like a clean toothbrush) to scrub the interior walls. For the heating element rod, gently use a cotton swab dipped in isopropyl alcohol. Wipe all surfaces with a dry microfiber cloth.
If That Didn't Work
If the smell is severe, the insulation around the heater may be contaminated. This cannot be cleaned safely. Discontinue use.
Heads Up
Never use water or liquid cleaners inside the chamber while the unit is plugged in or hot. You risk electrical shock and will cause more residue to bake on.