DIAGNOSIS The drive coupling is failing. The plastic gears that connect the motor to the blade are stripping. The burning smell is plastic friction.
FIX Stop using it immediately. Unplug it. This is a base problem. The internal drive mechanism is damaged. You cannot fix this yourself. The power base needs to be replaced. Contact NutriBullet for warranty or replacement.
IF THAT DIDN'T WORK If a new base makes the same noise, the extractor blade assembly itself has a seized bearing. Replace the blade assembly with a new, genuine part.
HEADS UP This is often caused by overloading the cup with hard, frozen items or running it too long. Follow the max fill lines. Do not blend ice cubes alone for extended periods.